P0121, P0122, P0123: Throttle Position Sensor Codes — Diagnosis Guide

Cable Throttle vs Drive-by-Wire

The diagnostic approach depends on what type of throttle control your vehicle has:

Cable throttle (pre-2005 most vehicles): A physical cable connects the gas pedal to the throttle body. There is a single TPS mounted on the throttle shaft. The sensor is often externally replaceable — you can buy just the TPS and bolt it on.

Drive-by-wire / Electronic Throttle Control (ETC): There is no cable. The gas pedal has its own position sensor (APP sensor), and the throttle body has an electric motor that opens and closes the plate. The TPS is built into the throttle body and is not separately replaceable on most applications. Drive-by-wire systems use dual TPS sensors (TPS1 and TPS2) for redundancy. TPS2 typically reads the inverse of TPS1 — when TPS1 goes up, TPS2 goes down. The PCM compares them to verify accuracy.

The Unplug Test

For P0123 (high voltage): Unplug the TPS connector. If the voltage drops to near 0V and now sets P0122 (low), you just proved the wiring and PCM are fine — the sensor is outputting too much voltage. Replace it.

If you unplug the TPS and the scan tool STILL shows high voltage — the signal wire is shorted to the 5V reference or to B+ voltage somewhere in the harness. The sensor is not even in the circuit anymore, so it cannot be the source. Find the short.

For P0122 (low voltage): Unplugging creates an open circuit, which mimics the same condition. Instead, backprobe the connector (harness side) with the sensor unplugged. You should have 5V reference on one pin and ground on another. If 5V reference is missing, check if other sensors sharing that 5V circuit also have issues — a shorted sensor elsewhere on the 5V reference line can pull down the reference for everyone.

The Voltage Sweep Test

This is the gold standard for P0121 (range/performance) diagnosis. It catches dead spots that the unplug test cannot find.

1. Connect a scan tool and monitor TPS voltage (or use a DVOM backprobed on the signal wire)
2. Very slowly move the throttle from fully closed to fully open over about 10 seconds
3. Watch the voltage — it should increase smoothly and linearly from about 0.5V to about 4.5V with zero dropouts, jumps, or flat spots
4. Then slowly close the throttle — the voltage should decrease just as smoothly

What to look for:

• Dead spot: The voltage sticks at one value while the throttle is still moving, then jumps to catch up. This is a worn resistive track inside the sensor — the wiper has lost contact in that zone.
• Dropout: The voltage drops to 0V momentarily then returns. This is an intermittent open in the signal circuit — often a cracked solder joint inside the sensor or a loose connector pin.
• Spike: The voltage jumps to 5V momentarily. This is an intermittent short to the reference circuit.
• Stepping: The voltage increases in small steps rather than smoothly. This can indicate a contaminated or worn resistive element.

A scope makes this test much more revealing than a scan tool because scan tools sample at 2-10 times per second while a scope captures thousands of samples per second. A 5-millisecond dropout that causes a hesitation will show on a scope but might be invisible on a scan tool.

Dual TPS Sensors (Drive-by-Wire)

On drive-by-wire vehicles, there are two TPS signals. The PCM compares them for validation:

• TPS1: Typically starts at ~0.5V closed and goes to ~4.5V open
• TPS2: Typically starts at ~4.5V closed and goes to ~0.5V open (inverse of TPS1)

At any throttle position, TPS1 + TPS2 should equal approximately 5.0V. If the sum drifts outside the PCM's tolerance, it sets P0121 and goes to reduced power mode. This is the most common cause of P0121 on modern vehicles — one of the two internal sensors has drifted, and the PCM catches the disagreement.

Monitor both TPS1 and TPS2 on your scan tool. If one tracks normally and the other is erratic or offset, the throttle body assembly needs to be replaced.

Step-by-Step Diagnosis

Step 1: Read Freeze Frame Data

Check what RPM, speed, and load the code set at. If it set at idle, focus on the closed throttle position. If it set during acceleration, the problem is in the mid-range sweep. Freeze frame data narrows your target.

Step 2: Check TPS Voltage at Key On Engine Off (KOEO)

TPS should read ~0.5V with the throttle fully closed. If it reads 0V, you have an open or ground issue. If it reads 1.2V+, either the throttle plate is not fully closing (carbon buildup) or the sensor has shifted.

Step 3: Voltage Sweep Test

Slowly open and close the throttle while monitoring voltage. Look for the dead spots, dropouts, and spikes described above.

Step 4: Check for Carbon Buildup

On cable throttle vehicles, carbon on the throttle bore prevents full closure. Clean the throttle body, perform an idle relearn, and clear the code before condemning the sensor.

Step 5: Check 5V Reference Circuit

If 5V reference is missing or low, check other sensors on the same 5V reference line. A shorted MAP sensor, for example, can pull the 5V reference down to 1V and cause TPS codes simultaneously. Fix the shorted sensor and the TPS code goes away. This is a common misdiagnosis — techs replace the TPS when the real problem is a shorted MAP sensor on the same reference circuit.

Pattern Failures by Make

Repair Costs