P0891 Code: TCM Power Relay Sense Circuit High

What the P0891 code means

The transmission control module (TCM) needs a steady power supply. Some vehicles feed it through a relay that the module itself controls, and others feed it directly from the fuse box. The module keeps watching its supply voltage.

The module monitors the control circuit through its own driver transistor. The P0891 code is set when that circuit shows high voltage, which usually means an open circuit or a short to voltage. Before you replace any part, check the fuse, relay, connector and wiring. Those are the most common causes.

When the voltage is low or unstable, the transmission shifts erratically or goes into limp mode, and solenoid codes can appear even though the solenoids are good. Check the battery, the alternator, the fuses and the grounds first.

To confirm the fault, measure the continuity of each wire between the component connector and the computer connector with the key off. It should be under 1 ohm, and an infinite reading means the wire is open. If the wiring is good, measure the component itself. An infinite resistance confirms it is open inside.

P0891 code symptoms

• Check engine light or another warning light on.
• Transmission in limp mode or shifting erratically.
• Erratic shifts, with gears that come at the wrong time or not at all.
• Limp mode, usually stuck in a single gear.
• Several transmission codes stored at the same time.

Likely causes of the P0891 code

1. Loose or broken connector. The most frequent cause, especially right after another repair.
2. Open coil or motor. The resistance reads infinite.
3. Cut control wire between the component and the computer.
4. Short to 12 volts. The control wire touches another wire that carries voltage.

How to diagnose the P0891 code step by step

1. Command the component with the scan tool and check whether it responds.
2. Check the fuse and the relay. Measure voltage on both sides of the fuse, and swap the relay with an identical one.
3. Measure the supply at the connector. You should see battery voltage with the key on or when the component is commanded.
4. Measure the resistance of the component and compare it with the manual. Also check its insulation to ground.
5. Check the control wire. Confirm continuity all the way to the computer and make sure there is no short.
6. Measure battery voltage and charging voltage.
7. Measure the voltage drop across the module grounds. It should be under 0.2 volts.

How to fix the P0891 code

• Repair the connector, the wiring or the power supply feed.
• Replace the transmission computer power relay or its supply component if it does not respond even with correct power and control signals.
• Clear the codes and road test the vehicle to verify the repair.

Technical reference data

• Supply voltage: battery voltage, which is 13.5 to 14.7 volts with the engine running.
• Wire continuity between connectors: below 1 ohm.
• Approximate values only: check the manufacturer data for your vehicle.

Brand-specific notes

Check the connector before anything else. A plug that is loose, cracked or not fully latched is the first thing to rule out when this code shows up after a repair.

P0891 on Chrysler

On many Chrysler vehicles the transmission relay sits in the fuse box in the engine compartment.

P0891 on Ford

On many Ford vehicles the transmission relay sits in the fuse box in the engine compartment.

Frequently asked questions about P0891