Damaged, chafed or shorted-to-ground wiring in the MIL request circuit between the TCM and ECM; Corroded, loose or pushed-back terminals in the TCM or ECM connectors; Low battery voltage or poor power and earth supply to the TCM; Water ingress or damage at the gearbox harness connector; Faulty TCM with an internal driver fault; Faulty ECM or an outdated control unit calibration affecting the request signal.

P0898

Transmission Control System MIL Request Circuit Low.

Generic
Powertrain
Generic code
Medium severity

How this code is built

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

What it means

P0898 means the transmission control module (TCM) has seen a low signal on the circuit it uses to request the malfunction indicator lamp (MIL) from the engine control module (ECM). The TCM monitors this request circuit and sets the code when the voltage is lower than it expects for the commanded state. This points to a wiring, connector or module communication fault rather than a mechanical gearbox fault, though the exact circuit layout varies by model and gearbox.

Symptoms

Malfunction indicator lamp on
Transmission warning light or gearbox fault message
Harsh, delayed or erratic gear changes
Gearbox stuck in limp mode or a fixed gear
Other transmission codes stored alongside P0898

Possible causes

Damaged, chafed or shorted-to-ground wiring in the MIL request circuit between the TCM and ECMlikely
Corroded, loose or pushed-back terminals in the TCM or ECM connectorslikely
Low battery voltage or poor power and earth supply to the TCMpossible
Water ingress or damage at the gearbox harness connectorpossible
Faulty TCM with an internal driver faultpossible
Faulty ECM or an outdated control unit calibration affecting the request signalrare

How to diagnose

Read all stored codes and freeze frame data, noting any network, power supply or other transmission codes, then check for relevant manufacturer bulletins for the model
Check battery condition and charging voltage, as low supply can cause spurious circuit codes
Visually inspect the TCM, ECM and gearbox harness connectors and wiring for chafing, corrosion, water and bent or loose pins
Check the TCM power feeds and earths for good voltage and low resistance
Using the wiring diagram for the vehicle, identify the MIL request circuit and test it for a short to ground, open circuit and correct voltage with the ignition on
Wiggle-test the harness while watching live data or circuit voltage to find intermittent faults
If wiring and supplies are good, check the ECM and TCM for software updates and only then consider testing or replacing the TCM

How to fix

Repair or replace the damaged section of wiring and secure it clear of sharp edges and heat
Clean or replace corroded connectors and terminals and re-seat them properly
Restore correct battery voltage and repair any poor power or earth connections
Dry out and seal or replace a water-affected gearbox connector or harness
Replace the TCM if testing confirms an internal fault, and code it to the vehicle as required
Update the control unit calibration if an outdated one is a known cause for that model

Can I keep driving?

Medium severity

The car can usually be driven short distances, but the gearbox may go into limp mode and the MIL will stay on, which could hide other faults. Get it diagnosed soon, and stop driving if the gearbox behaves erratically or shifts harshly.

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