Diagnosing Electric Power Steering Faults

Diagnosing EPS Faults — Electric Power Steering Troubleshooting Guide

Written by Anthony Calhoun, ASE Master Tech A1-A8

What Electric Power Steering Actually Does

Electric power steering replaces the hydraulic pump, fluid reservoir, high-pressure hoses, and rack with a single electric motor that applies steering assist directly to the column, pinion, or rack assembly. The system reads how hard the driver is turning the wheel through a torque sensor, reads vehicle speed and other conditions from the CAN bus, and the EPS control module calculates exactly how much motor current to apply. The result is variable assist — heavy assist at low speed for parking, light assist at highway speed for feel and stability — without a belt-driven pump running all the time.

This is not a complicated concept, but the diagnostic path is different from anything you learned on hydraulic systems. There is no fluid to check, no pressure to test, and no pump to listen to. What you have instead is a motor, a torque sensor, a steering angle sensor, a control module, and a CAN bus network tying it all together. When any of those pieces fail or lose communication, the driver feels it immediately in the steering wheel.

Understanding which type of EPS you are working on matters before you start diagnosing. There are three main architectures in production today.

Column-Mounted EPS (C-EPS)

The electric motor is mounted directly on the steering column. The motor applies torque to the column shaft, upstream of the steering gear. This design is common on smaller and lighter vehicles because the motor does not need to be as powerful — the mechanical advantage of the column and gear multiplies the motor output. General Motors used this design extensively on the Cobalt, HHR, Equinox, and early Cruze. Ford used it on the Fiesta and early Focus. It is inexpensive to manufacture and easy to package, but the motor and torque sensor are inside the cabin, which makes replacement more involved than it sounds on paper.

Pinion-Mounted EPS (P-EPS)

The motor drives the pinion gear directly, usually through a worm gear reduction. This puts the motor under the hood at the top of the steering gear. Toyota and Honda use pinion-assist designs on many of their mid-size cars and crossovers. The motor is exposed to heat and road contamination, and the worm gear creates a distinct noise signature when it starts to wear.

Rack-Mounted EPS (R-EPS)

Also called dual-pinion or rack-assist EPS. The motor drives a second pinion on the rack body itself, separate from the steering input pinion. This design handles higher loads and is used on heavier vehicles including trucks and large SUVs. Ford's current F-150 EPS and many European vehicles use this architecture. The advantage is that the motor works closer to the tire contact patch, reducing the torque required from the column. The disadvantage is cost and complexity — the rack and motor are typically replaced as an assembly.

EPS System Components

Before you can diagnose an EPS fault correctly, you need to know what you are looking at on any given vehicle. Every EPS system shares these core components, though their exact location and integration varies by manufacturer.

• Electric motor: Typically a brushless DC motor. On column-assist systems, it wraps around the column shaft. On pinion and rack-assist systems, it mounts at the gear. Current-generation motors are reliable, but they are sensitive to voltage supply problems and will thermal-shut down under sustained high-load conditions.
• Torque sensor: Measures the twist in the torsion bar between the upper and lower input shaft. This is the primary input the EPS module uses to determine assist level. Most use a contactless magnetic sensor. This is the single most common failure point in the entire EPS system.
• Steering angle sensor (SAS): Measures absolute steering wheel position. Located in the clock spring assembly or integrated with the torque sensor on many newer vehicles. Feeds data to the EPS module, stability control, and lane-keeping systems. Requires calibration after replacement or alignment work on most platforms.
• EPS control module: On most vehicles, integrated directly into the motor assembly. Processes torque sensor input, vehicle speed, engine RPM, and other CAN bus data to calculate the correct assist level. Controls motor current output. Stores diagnostic trouble codes.
• CAN bus network: EPS does not operate in isolation. It receives vehicle speed from the ABS module, engine status from the PCM, and yaw/lateral acceleration data from the stability control module. Lose that communication and the EPS module may reduce or eliminate assist entirely as a safety measure.

Common EPS Complaints and What They Mean

Customers will not walk in and tell you the torque sensor failed. They will tell you what they felt. Here is how to translate the most common complaints into a starting point.

Customer Complaint	Most Likely Cause	Starting Point
Heavy steering, no assist	EPS motor failure, module failure, power/ground loss, thermal shutdown	Scan for codes, check power and ground supply to EPS module
Intermittent assist loss	Torque sensor, connector corrosion, CAN bus dropout, thermal shutdown	Capture live data during failure, check for U-codes
Steering pulls to one side	Torque sensor offset, SAS miscalibration, internal motor fault	Check torque sensor live data at center, verify SAS calibration
EPS warning light on	Any stored DTC in EPS module — scan first before doing anything else	Full system scan including all modules
Grinding or whining noise	Worn worm gear (P-EPS), motor bearing, loose motor mounting	Isolate with stethoscope, compare noise to steering input angle
Notchy or rough steering feel	Intermediate shaft u-joint, motor cogging (rare), internal rack damage	Separate mechanical from electrical — does the notch follow steering input?

Diagnosing the Torque Sensor — Where Most EPS Failures Start

The torque sensor is the brain's primary input. If it sends bad data, the EPS module either provides wrong assist or shuts assist down entirely to protect the driver. In the real world, a failing torque sensor is responsible for the majority of EPS complaints that are not caused by wiring or communication faults.

Here is what the torque sensor does: the torsion bar connects the upper input shaft (what the driver turns) to the lower output shaft (what connects to the pinion or column gear). When the driver applies force to the steering wheel, the torsion bar twists slightly. The torque sensor measures that twist. More twist means the driver is working harder, which signals the EPS module to increase motor current and provide more assist.

A contactless magnetic torque sensor typically outputs two analog voltage signals that are mirror images of each other. At center, both signals sit at approximately 2.5 volts. As you turn left, signal one rises and signal two falls. Turn right, and they swap. The module monitors both signals and compares them — if they do not track correctly with each other, the module sets a code and may shut down assist.

How to Test the Torque Sensor With a Scan Tool

This is live data work. Pull up the EPS module data stream on your scan tool. Look for torque sensor voltage, torque sensor value (often displayed in newton-meters), or the raw voltage of both sensor channels depending on what the manufacturer exposes in their data stream.

With the vehicle on a flat surface, engine running, steering wheel centered:

1. Record the baseline torque sensor reading. It should be at or very near zero newton-meters, or the voltage should be at its midpoint.
2. Slowly turn the steering wheel left and right while watching the data. The sensor output should change smoothly and proportionally to your input. There should be no spikes, dropouts, or flat spots in the signal.
3. Pay attention to erratic readings — a sensor that jumps from 2.5V to 0.1V without corresponding wheel movement is failing. A sensor that reads the same value regardless of steering input is failed.
4. On platforms that expose both channels, verify they are tracking as mirror images. If channel one rises while channel two does not fall proportionally, the sensor is bad.

On GM column-assist vehicles (Cobalt, early Cruze, HHR), the torque sensor is integrated into the EPS motor/column assembly. When the torque sensor fails on these, you are typically replacing the entire column assembly. Verify with GM SPS programming requirements before ordering parts — the replacement module may require programming.

On Toyota applications (Camry, RAV4, Corolla), the torque sensor failure often presents as a C1511 or C1512 code along with a steering angle sensor fault. Toyota service information will walk you through checking the sensor output voltage at the EPS ECU connector before condemning the assembly. Pull up the wiring diagram and check voltage at the harness side first — connector corrosion at the EPS ECU is not uncommon on higher-mileage Toyotas.

Steering Angle Sensor Diagnosis and Calibration

The steering angle sensor tells the EPS module, the stability control module, and on newer vehicles the lane-keeping system exactly where the steering wheel is positioned at all times. It is an absolute position sensor, which means it needs to know its zero point — exactly where straight ahead is — before it can report accurate data.

Miscalibration is more common than outright sensor failure. Any time you perform an alignment, replace the steering gear, replace the clock spring, replace the EPS assembly, or disconnect the battery on certain vehicles, you may need to perform a steering angle sensor zero-point calibration. Skipping this step will leave the vehicle with an off-center steering feel, may trigger the EPS warning light, and on vehicles with electronic stability control, can cause the ESC to activate during normal straight-line driving because the system thinks the vehicle is turning when it is not.

Symptoms of SAS Miscalibration

• Steering wheel visually off-center after alignment even though alignment angles check out
• Stability control activating on a straight road
• EPS warning light on with a C1414 or similar SAS calibration fault code
• Drift or pull that only appears at highway speed

How to Calibrate the Steering Angle Sensor

The procedure varies by manufacturer, but the general process is consistent. You need a scan tool with bidirectional control capability for the specific vehicle. Drive the vehicle in a straight line to establish a baseline, then use the scan tool to execute the zero-point calibration function. Some manufacturers require specific conditions: engine running, vehicle stationary on level ground, steering wheel centered within a specific degree tolerance, and no steering input during the procedure.

Honda and Acura are particular about this. After replacing the EPS unit or performing a steering-related repair, you must use Honda's HDS tool to run the Motor Angle Learning procedure and the Torque Sensor Neutral Position Memorization. Skipping either step leaves the EPS operating with incorrect baseline data.

Ford requires a steering angle sensor reset through IDS/FDRS after replacing the steering gear on most current platforms. The reset function is in the EPS module section under special functions. Without it, the new rack will not know where straight ahead is.

EPS Motor Testing

The electric motor in an EPS system is not something you bench test in most shop environments. What you can test is the power supply to the motor, the ground quality, and the motor's resistance at the harness connector when the assembly is on the vehicle.

Power and Ground Supply

EPS motors draw significant current — anywhere from 20 to 80 amps under full assist load depending on the vehicle. A high-resistance connection in the power or ground circuit will cause voltage drop under load, which the module interprets as a system fault. Check power and ground at the EPS module/motor connector with the system under load (wheel turned against a solid stop, engine running). Anything more than 0.5 volts of voltage drop in the power feed or ground circuit is a problem that needs to be corrected before condemning the motor.

Motor Winding Resistance

With the EPS assembly disconnected, check resistance across the motor phase windings at the harness connector. Compare to manufacturer specifications. An open winding will read infinite resistance. A shorted winding will read significantly below specification. Either condition requires motor replacement — or full assembly replacement, depending on the vehicle.

Thermal Shutdown

This one catches technicians off guard. EPS motors have thermal protection built in. If the motor gets too hot — from extended parking lot maneuvering, repeated lock-to-lock turns during wheel alignment, or a customer who spent twenty minutes doing three-point turns — the module will reduce or eliminate assist until the motor cools down. The customer drives it in, the steering is heavy, it sits in the lot for an hour, and now it works fine. No codes. No fault.

If the customer complaint is heavy steering that goes away on its own and you cannot duplicate it, ask specifically about what the vehicle was doing just before the complaint occurred. Repeated low-speed turning is the giveaway. This is not a failure condition — it is a protection function operating normally. Explain it to the customer and document it.

CAN Bus Communication and EPS

EPS is not a standalone system. It needs data from other modules to function correctly. The EPS module uses vehicle speed data from the ABS/EBCM module to scale assist — full assist at zero mph, progressively less at highway speed. It monitors engine RPM to confirm the engine is running before enabling assist. On vehicles with active safety systems, it receives commands from the lane-keeping or driver assist modules.

When CAN bus communication is disrupted, the EPS module does not get the data it needs. Depending on the manufacturer's programming, it may default to a fixed assist level, reduce assist, or disable assist entirely. This is why you always want to do a full system scan on an EPS complaint, not just scan the EPS module. If you find U-codes alongside the EPS codes, the communication problem is likely upstream of the EPS system.

U-Code Examples to Look For

• U0100: Lost communication with ECM/PCM — EPS module cannot confirm engine running status
• U0121: Lost communication with ABS control module — EPS cannot receive vehicle speed data
• U0415: Invalid data received from ABS — data is arriving but is out of range

Diagnose the CAN bus fault first. Chasing an EPS motor or torque sensor when the real problem is a CAN bus open circuit or a failing ABS module sending bad vehicle speed data wastes time and parts. Fix the communication issue, clear codes, retest, and then evaluate whether any EPS-specific faults remain.

Manufacturer-Specific Issues Worth Knowing

General Motors — Column-Mounted EPS

The GM column-assist EPS on the Cobalt, HHR, G5, and first-generation Equinox is well-documented for two failure patterns. First, the motor connector corrodes. The 12-pin connector at the EPS motor develops oxidation that creates resistance in the signal circuits, leading to erratic torque sensor readings and intermittent assist loss. Clean the connector with electrical contact cleaner and apply dielectric grease before condemning the assembly. Second, outright motor failure. When the motor itself fails on these, the column assembly is replaced as a unit. Make sure you have the correct calibration procedure lined up — the replacement module requires SPS (Service Programming System) programming through GM's TIS2Web or equivalent. A replacement motor with no programming is a very expensive paperweight.

Toyota — Torque Sensor and Intermediate Shaft

Toyota EPS torque sensor failures are common enough to have their own pattern. The C1511 torque sensor code on 2007-2013 Camry and similar platforms often turns out to be the torque sensor itself or the EPS ECU. Toyota's diagnostic procedure walks you through checking specific voltages at the ECU connector to differentiate between the two. Do not skip that step — the EPS ECU and the torque sensor are priced very differently.

Separately, Toyota has a known issue on certain Corolla and Matrix models with the steering intermediate shaft developing a clunk or notchy feel during low-speed parking maneuvers. This is a mechanical issue, not an EPS electrical fault. The intermediate shaft has a slip joint that can wear or bind. Do not let an EPS code on a Toyota lead you to replace electrical components if the customer complaint is mechanical in nature — isolate the complaint first.

Honda and Acura — EPS ECU and Programming

Honda EPS failures tend to split between the torque sensor and the EPS ECU itself. The 2005-2010 Honda Odyssey had a documented pattern of EPS ECU failure, with the steering becoming heavy or the EPS light illuminating after a hard impact or extended high-load use. Honda released software updates for some of these, but when the ECU hardware fails, the replacement requires the Motor Angle Learning and torque sensor neutral position relearn procedures mentioned earlier. If you replace a Honda EPS unit and skip the relearn, the system will operate incorrectly and may store calibration fault codes immediately.

Ford — Rack Replacement vs. Gear Replacement

Ford's current EPS lineup on the F-150, Escape, Explorer, and Mustang uses rack-assist designs where the motor is integrated into the rack assembly. When the EPS motor fails on these vehicles, the entire steering gear assembly typically must be replaced — there is no serviceable motor-only option through Ford's parts system. This is an expensive repair, and the labor time is significant. Confirm the failure is in the motor/gear assembly and not in the power supply, grounds, or CAN bus before quoting the customer on a rack replacement. After installation, Ford IDS/FDRS is required to perform the EPS configuration and steering angle sensor reset. Without the scan tool procedure, the vehicle will not steer correctly.

Calibration and Relearn — When It Is Required

EPS calibration requirements catch technicians off guard because the trigger conditions are broader than most people expect. It is not just after replacing the EPS assembly. Here is when you need to verify or perform EPS calibration:

1. After EPS motor or assembly replacement: Always. Without exception.
2. After steering gear replacement: Always. The new gear does not know where center is.
3. After wheel alignment: On platforms where the SAS is not automatically corrected by the module. Check the vehicle's service information — this varies by make and model.
4. After clock spring replacement: The clock spring often contains or interfaces with the SAS. Replacement requires calibration on most platforms.
5. After battery disconnect: Some vehicles lose the SAS zero-point calibration when power is removed. Honda is particularly known for this. If a customer comes in with an EPS light after a battery replacement, SAS calibration is your first check.
6. After any steering column service: If you have disassembled the steering column for any reason on a vehicle with a column-assist EPS, verify calibration before returning it to the customer.

Safety During EPS Service

Electric power steering systems can apply steering force without any driver input when the motor is energized. This is not a theoretical concern — a motor that receives a command signal during testing will turn the steering wheel. Keep this in mind during any EPS service work.

Never reach into the steering column area with the ignition on and the EPS system enabled. If you need to access components on or near the column with the system powered, have a second technician monitoring the situation or disable the EPS system through the fuse before working in that area.

When removing an EPS motor from a column-assist application, secure the steering wheel so it cannot rotate. Use a steering wheel holder or lock the column with the ignition key. If the wheel rotates while the clock spring is disconnected or during motor removal, you risk damaging the clock spring or binding internal components.

During scan tool testing that involves commanding the EPS motor, make sure the vehicle is secured and no one is near the steering wheel or the steering gear linkage. The motor can move the steering gear rapidly, and a rotating steering wheel can cause injury if someone is in its path.

Always consult manufacturer service information for specific disassembly procedures and safety warnings before starting EPS service work on an unfamiliar platform. The general principles here apply broadly, but the exact connector locations, torque specifications, and programming requirements are vehicle-specific and must be confirmed before you start turning wrenches.

Final Diagnostic Approach

When an EPS complaint walks through the door, work in this order. First, do a full system scan before touching anything. Capture all codes across all modules. Look at the freeze frame data if codes are present. Identify any U-codes that indicate communication problems and address those first.

Second, pull up EPS live data and verify the basics: torque sensor reading at center, SAS reading at center, vehicle speed signal present, battery voltage to the module. You can eliminate or confirm a large number of failure modes in ten minutes with a scan tool before you pick up a wrench.

Third, verify calibration status. If codes suggest a calibration issue or if recent work was performed on the vehicle, perform the calibration procedure before condemning hardware.

Fourth, test the torque sensor with steering input. Watch the signal trace during slow steering inputs. Erratic signals confirm a sensor problem. Clean, proportional signals mean the sensor is likely intact and the problem is elsewhere.

Fifth, check power, ground, and wiring before replacing any assembly. A $1,200 EPS rack replacement that fails to fix a heavy steering complaint because the real problem was a corroded ground strap is a warranty job no one wants to do twice.

EPS systems are reliable when they are healthy and straightforward to diagnose when you work through them systematically. The technology is not complicated — it is just different from what most technicians learned on hydraulic systems. Master the scan tool data, understand the communication architecture, and verify calibration at every relevant touchpoint. That approach will get you to the right answer faster than any shortcut.