Ford 6.7 Power Stroke No Crank No Start — Diagnostic Walkthrough

Introduction

A no-crank no-start on a 6.7 Power Stroke Super Duty will humble you if you do not understand how the starting circuit works on these trucks. This is not your grandfather's diesel where battery voltage goes straight through the ignition switch to the starter solenoid. On the 6.7, the PCM is the gatekeeper. It decides if and when the starter engages, and it checks a list of conditions before it sends the command.

I have seen experienced techs spend entire days chasing a no-crank on one of these trucks because they started at the starter motor and worked backwards. That is the wrong approach. On the 6.7, you start at the PCM and work forward. If the PCM is not commanding the starter relay, it does not matter if the starter motor is brand new — it is not going to spin.

This article is the walkthrough I wish every tech had the first time they face this problem. It covers the complete starting circuit, what the PCM looks for, how to monitor the critical PIDs during a crank attempt, and what to do when you hit a dead end — including the internal PCM fault codes that can leave you completely stuck if you do not know they exist.

The Complete Starting Circuit

Here is the starting circuit on the 6.7 Power Stroke Super Duty, laid out in order from power source to starter motor:

Batteries: The 6.7 uses a dual-battery system wired in parallel. Both batteries are under the hood, connected together, providing approximately 12.6 volts with very high cranking amp capacity. Diesel engines have high compression ratios and need significantly more cranking power than gasoline engines.

Mega fuse: Battery voltage feeds through a high-amperage mega fuse to the battery junction box (BJB) — the main fuse panel in the engine compartment. This fuse protects the main power distribution circuit. If it blows, nothing works.

Battery junction box: Inside the BJB, the starter relay lives alongside other high-current relays. The relay has a power feed directly from the battery circuit (always hot) and a control circuit that comes from the PCM.

Starter relay: This is a standard automotive relay — power feed on pin 30, output to the starter on pin 87, ground control from the PCM on pin 85, and a coil power feed on pin 86. When the PCM provides a ground signal to pin 85, the relay coil energizes, closes the contacts, and sends battery voltage through pin 87 to the starter motor solenoid.

Starter motor: The starter solenoid receives voltage from the relay, engages the drive gear into the flywheel, and cranks the engine. On the 6.7, the starter is a gear-reduction unit mounted on the driver side of the engine.

The critical thing to understand: the ignition switch does NOT directly control the starter relay. The ignition switch sends a signal to the PCM saying "the driver wants to crank." The PCM then decides whether to energize the relay based on its input checks. This is fundamentally different from older vehicles where the ignition switch was in the power path.

The PCM's Role — Why It Controls Everything

Ford moved to PCM-controlled starter circuits for several reasons, and understanding the "why" helps you understand the diagnostic approach.

First, anti-theft. The PATS system requires the PCM to validate the key before allowing the engine to start. If the ignition switch directly powered the starter, a thief could bypass anti-theft by simply hot-wiring the starter circuit. With the PCM in control, there is no way to crank the engine without a valid key signal.

Second, transmission safety. The PCM verifies the transmission is in Park or Neutral before commanding the starter. This prevents the truck from lurching forward if someone tries to start it in gear. On older vehicles, the neutral safety switch was in the starter circuit itself — if it failed, the truck would not crank in any gear. On the 6.7, the transmission range sensor feeds the PCM, and the PCM makes the decision.

Third, system protection. The PCM monitors battery voltage, network communication status, and its own internal health before commanding the starter. If system voltage is too low, the PCM may not command the starter to prevent a brownout that could corrupt module memory. If there is a network communication failure, the PCM may inhibit cranking because it cannot verify all inputs.

This is why "start at the PCM and work forward" is the right approach. The PCM is making the decision. If it is not commanding the starter relay, you need to find out why — and the PCM will usually tell you through its data PIDs and stored codes, if you know where to look.

Inputs the PCM Checks Before Cranking

Here is the checklist the PCM runs through before it will ground the starter relay control circuit:

1. PATS — Passive Anti-Theft System: The PCM must receive a valid key transponder signal. On trucks with a traditional key, the PATS transceiver in the steering column reads the transponder chip in the key and sends the ID to the PCM. On trucks with push-button start, the BCM (Body Control Module) reads the key fob and communicates with the PCM over the CAN network. If the PCM does not see a valid key, no crank. The theft indicator light on the dash is your clue — a rapidly flashing theft light means PATS has not validated.

2. Transmission Range Sensor (TRS): The digital TRS must read Park or Neutral. If the sensor reads any other gear, or if the signal is out of range, the PCM will not command the starter. A failed TRS can read an invalid state that the PCM interprets as "not in park, not in neutral" — even though the transmission is physically in park. This is a very common cause of intermittent no-crank conditions.

3. Ignition Switch Position: The PCM needs to see the ignition switch input transition to the Crank position. This signal comes through the BCM on most model years. If the ignition switch is worn or the BCM does not see the crank request, the PCM never gets the command.

4. Brake Pedal (push-button start models): On trucks with push-button start, the brake pedal must be depressed. The brake switch signal goes to the BCM and then to the PCM. A failed brake switch can prevent cranking.

5. System Voltage: The PCM monitors its own supply voltage. If voltage is below a minimum threshold — which can happen with one or both dead batteries — the PCM may not command the starter because it cannot guarantee stable operation during the voltage drop that occurs when the starter engages.

6. Internal Health: The PCM performs a self-check. If it has detected an internal hardware failure (stored as U0601 or similar internal codes), it may inhibit starter operation because it cannot trust its own outputs.

Step-by-Step Diagnostic Walkthrough

Here is the exact process I follow every time a no-crank 6.7 Power Stroke comes into the bay. Follow it in order. Do not skip steps.

Step 1 — Battery voltage. Before you touch a scan tool, put a voltmeter on both batteries. Each battery should read 12.4V or higher with the key off. If either is below 12.0V, charge both batteries fully before proceeding. I cannot overstate this — low voltage causes bizarre symptoms on the 6.7 because the PCM and BCM start behaving unpredictably when supply voltage drops. I have seen no-crank conditions that were 100% caused by one weak battery pulling the system down. Load-test both batteries. Replace any that fail.

Step 2 — Scan for codes. Connect your scan tool and read codes from ALL modules — not just the PCM. Check the PCM, BCM, TCM, and instrument cluster. Look for U-codes (network communication), B-codes (body/PATS), and P-codes. Write down everything. Do not clear anything yet.

Step 3 — Check the theft light. Turn the key to the ON position (do not attempt to crank). Watch the theft indicator light on the dash. It should illuminate briefly and then go out — this means PATS has validated the key. If it flashes rapidly or stays solid, PATS has not validated. Try a second programmed key. If the second key works, the first key's transponder has failed. If neither key works, the PATS transceiver, the wiring, or the PCM's PATS module is the issue.

Step 4 — Monitor PCM PIDs during crank attempt. This is where you find the answer. With the scan tool connected, pull up the following PIDs and watch them in real time while an assistant turns the key to the crank position:

- Starter Relay Command (STR_CMD or similar): Does the PCM command the relay ON when the key goes to crank? If yes, the PCM is doing its job and the problem is downstream — relay, wiring, or starter. If no, the PCM is refusing to command the relay and you need to find out which input is blocking it.

- Ignition Switch Position (IGN_SW): Does the scan tool show the switch going to the Crank position? If the PCM never sees the crank request, it will never command the relay.

- Transmission Range (TR_PRNDL or TR_SIGNAL): Does the scan tool show Park or Neutral? If it shows an invalid state, gear position, or out-of-range value, the TRS is the problem.

- PATS Status: Does the PCM show the key as validated? Look for PATS parameters — key number, key status, validation status.

Step 5 — Test the starter relay. If the PCM IS commanding the relay (Step 4 confirmed the command), but the engine still does not crank, the problem is in the relay or starter circuit. Pull the starter relay from the BJB. With the relay out, back-probe the socket: you should have battery voltage on the power feed pin. Reinstall the relay (or a known-good substitute) and back-probe the output pin while an assistant cranks. You should see battery voltage on the output going to the starter solenoid. If you have voltage out of the relay but the starter does not crank, the starter motor or its ground circuit has failed.

Step 6 — If the PCM is NOT commanding the relay. Go back to the PIDs from Step 4. Whichever input is wrong — ignition switch, TRS, PATS — that is your direction. The scan tool data will tell you which input the PCM is missing. Fix that input and retest.

U0601 — Internal PCM Fault

This is the one that ends careers if you do not know about it. U0601 is an internal control module performance code. It does not mean the PCM cannot communicate with other modules on the network — that is a U0100 or U0101. U0601 means the PCM has detected a failure within its own internal hardware.

Think of it this way: the PCM runs self-diagnostic checks on its own processors, memory, and output drivers. If one of those checks fails, it stores U0601. Depending on which internal component failed, the PCM may lose specific functions — and one of those functions can be the starter relay output driver.

Here is the scenario: truck rolls in, no crank, no codes in the PCM except U0601. The PCM is powered, it communicates on the scan tool, it shows all inputs as valid — ignition switch in crank, TRS in park, PATS validated — but it will not command the starter relay. Every input checks out but the output is dead. That is U0601. The PCM has an internal hardware failure that is preventing it from commanding the starter relay driver.

If U0601 is current — not historical — and you have confirmed a loss of PCM function (no starter command with all inputs valid, or loss of injector pulse, or loss of a specific network), the PCM needs to be replaced. This is not a reflash issue. This is not a software update. The internal hardware has failed. A new or remanufactured PCM, programmed to the vehicle, is the fix.

I have to be specific about "current versus historical" because U0601 can set momentarily during a brownout event (like a weak battery during cranking) and then not re-set after the battery is charged. If U0601 is historical and the PCM is functioning normally now, it may have been a one-time event from low voltage. But if it is current, repeatable, and accompanied by a confirmed loss of function, the PCM is done.

This exact scenario caught a friend of mine recently. Dead F-550, no crank, all inputs checked good, no other codes. He spent two days checking wiring, swapping relays, testing the starter. The whole time, U0601 was sitting right there in the code list, and he did not know what it meant. The PCM had an internal failure on the starter relay output driver. New PCM, programmed, truck started on the first key turn. Two days of labor that should have been two hours.

Common Diagnostic Traps

After 25 years of diagnosing no-crank conditions on diesel trucks, here are the traps I see techs fall into on the 6.7 Power Stroke:

Trap 1: Starting at the starter motor. On older trucks, this made sense. On the 6.7, the starter is almost never the problem on a true no-crank. The control circuit — the PCM's decision to energize the relay — is where the failure lives 80% of the time. Start at the PCM. Check the data. Then work forward.

Trap 2: Ignoring the second battery. Techs check one battery, see 12.5V, and move on. But the other battery is at 10.8V and dragging the entire system down during the crank attempt. Both batteries. Every time. No exceptions.

Trap 3: Not monitoring PIDs during the crank attempt. You cannot diagnose this problem with the key in the ON position. You need to see what the PCM is doing — and not doing — at the moment the key goes to crank. That requires a scan tool connected and recording while an assistant turns the key. Static code reading is not enough.

Trap 4: Clearing codes before reading them. I have had trucks towed in from other shops where the previous tech cleared all codes "to start fresh." Now I have no history, no U0601, no PATS codes, nothing. Read everything first. Write it down. Photograph the screen. Then decide what to clear.

Trap 5: Overlooking the transmission range sensor. The TRS on the 6.7 is a digital sensor, and when it fails, it can report an impossible gear state that the PCM does not recognize as Park or Neutral. The truck is physically in Park, the shifter is in Park, but the PCM's TRS PID shows something invalid. This is an incredibly common cause of intermittent no-crank that drives fleet managers crazy. "It would not start this morning, but it started fine this afternoon." The TRS was reading wrong when it was cold and reading correctly once it warmed up.