TCM Engine Speed Output Circuit Low
The transmission control module isn't receiving a proper engine speed signal, like a tachometer needle stuck too low. This causes the transmission to lose critical timing information needed for smooth shifting.
The TCM monitors engine RPM feedback from the ECM via CAN bus or dedicated signal lines to synchronize shift timing and torque converter lockup. The signal voltage should fluctuate within normal operating range; a persistently low or absent signal triggers this fault. The TCM expects consistent RPM data updates to correlate engine load with transmission demand.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Engine speed signal voltage | 0.5-4.5V (variable with RPM) | Below 0.2V or no signal for >500ms |
| CAN bus engine speed message | Updated every 10-50ms | Missing or delayed >100ms |
Code P0738 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0738 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.