Faulty TCM power relay with contacts stuck closed or internal fault; Short to voltage in the relay sense wiring, or chafed harness; Corroded, damaged or loose connector terminals at the relay, fuse box or TCM; Charging system fault or battery over-voltage affecting the sense reading; Water ingress or damage in the fuse and relay box; Internal TCM fault, or a TCM needing updated software where the manufacturer has identified this as a known cause.

P0891

TCM Power Relay Sense Circuit High.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
8: AreaTransmission
91: FaultThe specific fault in that area

What it means

P0891 means the transmission control module (TCM) has detected a voltage that is too high on the sense circuit that monitors its power relay. The TCM compares the voltage it sees on the sense line with what it expects for the commanded relay state, and sets the code when the reading is higher than it should be, for example voltage present when the relay should be off. The exact relay layout and monitoring strategy vary by vehicle, so check model-specific information.

Symptoms

Transmission warning light or malfunction indicator lamp on
Transmission stuck in a fixed gear (limp mode)
Harsh, delayed or erratic gear changes
No shifting or loss of some gears
Gearbox may not respond after restarting the engine
Reduced performance or poor economy

Possible causes

Faulty TCM power relay with contacts stuck closed or internal faultlikely
Short to voltage in the relay sense wiring, or chafed harnesslikely
Corroded, damaged or loose connector terminals at the relay, fuse box or TCMpossible
Charging system fault or battery over-voltage affecting the sense readingpossible
Water ingress or damage in the fuse and relay boxpossible
Internal TCM fault, or a TCM needing updated software where the manufacturer has identified this as a known causerare

How to diagnose

Read all stored and pending codes and note any other transmission, power supply or communication codes
Review freeze frame data to see when the fault set, including engine speed, vehicle speed and battery voltage
Check the battery and charging voltage with the engine off and running to rule out over-voltage or an unstable supply
Inspect the relay, fuses and fuse box for corrosion, water, heat damage or loose terminals
Inspect the TCM and relay sense wiring and connectors for chafing, pin damage and shorts to voltage
Using the wiring diagram, test the sense circuit for unwanted voltage with the relay off and compare with the expected value when on
Test the relay itself, or swap it with a known good identical one if the manufacturer allows
Check for technical service bulletins or software updates for the specific model and gearbox

How to fix

Replace the TCM power relay if it is faulty or sticking
Repair or replace damaged sense wiring that is shorted to voltage
Clean or replace corroded or damaged connectors and terminals
Correct any charging system fault or replace a failing battery
Dry out and repair or replace a water-damaged fuse and relay box
Repair or replace the TCM, or update its software, only if testing confirms a fault or a known manufacturer update applies

Can I keep driving?

Medium severity

The car can usually be driven carefully a short distance, but the gearbox may be in limp mode with limited gears. Get it checked soon, as erratic shifting could worsen or leave you stranded.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.