What it means
Same as P0737 with the output shorted to ground. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
At a glance
- Usually lights the check-engine light
How it works
The TCM sends engine-speed information to other modules through an output circuit. P0738 sets when that output is detected low, commonly because the signal line is shorted to ground or has a poor connection that pulls the signal down.
Common causes
Most likely first.
- Output line shorted to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
How to diagnose it
- Record freeze-frame data and scan other modules for missing engine-speed or communication codes to see which systems are affected.
- Check whether engine-speed data appears plausible in the engine controller and TCM, separating a bad source reading from a fault on the TCM output.
- Inspect the TCM output harness and connector for chafed insulation, water intrusion, corrosion, or pushed-back pins, especially where the harness moves or bends.
- With live data or the appropriate circuit test active, gently move the harness to see whether the low-output fault changes, then isolate any short to ground.
- Test circuit continuity and shorts and verify TCM power and ground against the wiring diagram and manufacturer's specification before considering an internal TCM fault.
Common fixes
- Repair chafed insulation where the engine-speed output wire is shorting to ground.
- Clean, secure, or repair connector pins with corrosion or poor contact.
- Repair a damaged harness section or water intrusion at a connector seal.
- Correct unstable TCM power or ground connections.
- Replace the TCM only if testing confirms the output remains low with the external circuit verified sound.
Can I keep driving?
Avoid towing and hard driving until the low output signal is diagnosed, especially if shifting becomes harsh or limp mode occurs. A continued circuit fault can disrupt other modules' use of engine-speed data and may lead to unpredictable transmission operation.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.