P0738: TCM Engine Speed Output Circuit Low
The P0738 code indicates that the Transmission Control Module (TCM) has detected a low voltage or signal in the engine speed output circuit. This circuit is responsible for communicating engine RPM data from the Engine Control Module (ECM) to the TCM for proper transmission shifting. A low signal can cause erratic shifting, limp mode, or transmission damage if ignored.
01. Most Common Causes
- Faulty engine speed sensor (crankshaft position sensor)
- Wiring or connector issues in the engine speed output circuit
- Malfunctioning TCM or ECM
- Corroded or loose electrical connections
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Erratic or harsh transmission shifting
- Transmission stuck in limp mode or failsafe mode
- Poor acceleration or hesitation
03. Step-by-Step Diagnosis
Step 1: Scan for additional codes and record freeze frame data.
Step 2: Inspect the engine speed sensor and its wiring for damage, corrosion, or loose connections.
Step 3: Test the engine speed sensor signal using a multimeter or oscilloscope.
Step 4: Check for TCM/ECM software updates or reprogramming needs.
Step 5: If all else fails, test the TCM and ECM for internal faults.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
No, it is not recommended. Driving with P0738 can cause transmission damage, erratic shifting, or sudden loss of power, leading to unsafe driving conditions.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for diagnosis and repair, depending on the root cause. Sensor replacement may cost $100-$200, while TCM/ECM repairs can be higher.