P0738 TCM Engine Speed Output Circuit Low

P0738 indicates the TCM engine speed output circuit is low. The output signal is below the expected level or unavailable when requested.

Code details

Generic OBD-II · P0

System: Transmission
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not call the controller bad until the low circuit has been isolated from wiring and connector faults.

Symptoms of P0738

Harsh, delayed, or slipping shifts, or being stuck in one gear.
Check-engine light, sometimes with limp-home mode.
Flaring RPM or a shudder under load.

Common causes of P0738

Low or degraded transmission fluid.
A failing speed sensor, shift solenoid, or pressure-control circuit.
Wiring, connector, or internal hydraulic/mechanical faults.

Useful checks before replacing parts

Measure the engine-speed output circuit for low voltage, short-to-ground, open circuit, and poor terminal contact.
Compare engine RPM data across modules and check whether the TCM receives valid speed information before outputting it.
Inspect TCM connector pins, related fuses, grounds, splice points, and network faults that can mimic a low output.

Related OBD-II codes

These nearby transmission entries can help compare wording and pin down the fault area.