Faulty or sticking TCM power relay; Open circuit, high resistance or short in the relay sense or power supply wiring; Corroded, loose or damaged connectors or terminals at the relay, fuse box or TCM; Blown or poor-contact fuse feeding the TCM or relay; Low battery voltage or poor earth connections; Internal fault in the TCM (or combined mechatronic unit, where fitted); Outdated TCM software where the manufacturer has issued a known fix.

P0888

TCM Power Relay Sense Circuit.

Generic
Powertrain
Generic code
Medium severity

How this code is built

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

What it means

P0888 is the generic code for a fault in the Transmission Control Module (TCM) power relay sense circuit. The TCM monitors the feedback signal from its power relay and sets this code when the voltage it sees does not match what it expects for the commanded relay state, for example no voltage when the relay should be on, or voltage present when it should be off. Relay location, wiring and strategy vary by manufacturer and model.

Symptoms

Gearbox warning light or malfunction indicator lamp on
Gearbox stuck in one gear (limp mode) or restricted gear selection
Harsh, delayed or erratic shifts
No shift or no drive in some cases
Intermittent loss of gearbox function that comes and goes
Other gearbox or network fault codes stored alongside

Possible causes

Faulty or sticking TCM power relaylikely
Open circuit, high resistance or short in the relay sense or power supply wiringlikely
Corroded, loose or damaged connectors or terminals at the relay, fuse box or TCMlikely
Blown or poor-contact fuse feeding the TCM or relaypossible
Low battery voltage or poor earth connectionspossible
Internal fault in the TCM (or combined mechatronic unit, where fitted)rare
Outdated TCM software where the manufacturer has issued a known fixrare

How to diagnose

Read all stored codes and freeze frame data, noting whether the fault is current or intermittent and what other codes are present
Check battery condition and charging voltage, as low or unstable voltage can set relay-related codes
Visually inspect the relevant fuses, the relay and its socket, and the TCM connector for corrosion, water ingress, heat damage or pushed-out pins
Check the wiring diagram for your model to identify the relay, its supply, earth and sense wire
Test the relay supply and earth, and check the voltage on the sense circuit with the relay commanded on and off
Wiggle-test the harness while monitoring live data to find intermittent faults
Check wiring for continuity and for short to earth or to supply, with the battery disconnected and the connectors unplugged
Check for manufacturer software updates or technical bulletins for your model before considering TCM replacement

How to fix

Replace the faulty TCM power relay
Repair or replace damaged wiring, and clean or renew corroded or loose connector terminals
Replace the blown fuse after finding out why it failed
Repair poor battery supply or earth connections and restore proper charging voltage
Update the TCM software if an outdated calibration is a known cause for that model
Repair or replace the TCM or mechatronic unit only after wiring and relay faults are ruled out

Can I keep driving?

Medium severity

The car may go into limp mode or lose drive without warning, so limit driving to getting it to a workshop soon. Stop driving if the gearbox will not shift properly or drive is lost, as this can be unsafe in traffic.

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