P0888 Code: TCM Power Relay Sense Circuit

What the P0888 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 monitors its supply voltage all the time.

The module watches the control circuit through its own driver transistor. The P0888 code is set when it detects a fault in the power or the ground of that circuit. 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 enters limp mode, and solenoid codes can appear even though the solenoids are good. Check the battery, the alternator, the fuses and the grounds first.

Measure the voltage at the connector with the component working. A big drop compared with the battery points to a power feed or a ground with too much resistance. Measure the voltage drop on the ground wire as well. It should be under 0.2 volts under load.

P0888 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 P0888 code

1. Blown fuse or relay. The fuse may be open, or the relay contacts may be worn.
2. Loose or corroded ground. It adds resistance to the circuit.
3. Damaged power wire. Heat or rubbing wears through the insulation.

How to diagnose the P0888 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 P0888 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.
• Voltage drop across a ground under load: below 0.2 volts.
• Approximate values only: check the manufacturer data for your vehicle.

Brand-specific notes

Ground problems are easy to miss because the wire looks fine. Clean every ground point down to bare metal and retest before you spend money on parts.

P0888 on Chrysler

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

P0888 on Ford

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

Frequently asked questions about P0888