P0738 - TCM Engine Speed Output Circuit Low
The P0738 code indicates that the Transmission Control Module (TCM) has detected a low voltage condition 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 shift scheduling and torque converter lockup. A low voltage signal can cause erratic shifting, harsh engagement, or limp mode. Common causes include a faulty engine speed sensor, damaged wiring or connectors, a failing TCM, or an ECM issue.
1. Symptoms You Will Notice
- Check Engine Light illuminated
- Erratic or harsh shifting
- Transmission stuck in limp mode or fails to shift
- Poor acceleration or reduced fuel economy
2. Most Common Causes
- Faulty engine speed (RPM) sensor
- Damaged or corroded wiring/connectors in the sensor circuit
- Faulty TCM or ECM
- Short to ground in the signal circuit
3. Step-by-Step Diagnosis
Step 1: Scan for additional codes and record freeze frame data.
Step 2: Visually inspect the engine speed sensor and its wiring for damage, corrosion, or loose connections.
Step 3: Test the sensor's resistance and voltage output with a multimeter per manufacturer specs.
Step 4: Check for shorts or opens in the circuit between the sensor and TCM/ECM.
Step 5: If wiring and sensor are okay, test TCM/ECM operation or consult a professional.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances as the transmission may shift erratically or enter limp mode, potentially causing further damage.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for a sensor replacement, but can be higher if wiring repairs or TCM replacement is needed.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.