P0891 TCM Power Relay Sense Circuit High

P0891 indicates a high TCM power relay sense circuit signal. The relay feedback voltage is higher than expected or present when it should not be.

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 replace the TCM until the relay sense circuit is checked for backfeed or stuck relay contacts.

Symptoms of P0891

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 P0891

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

Check for short-to-voltage, stuck relay contacts, incorrect relay installation, damaged wiring, and sense circuit faults.
Compare relay command with relay output and sense voltage when the relay should be off and on.
Inspect aftermarket wiring, fuse-panel corrosion, and repaired harness sections that can backfeed the sense circuit.

Related OBD-II codes

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