The transmission control relay supplies power from the engine’s electrical system to the shift solenoids when you start the engine. A P0891 DTC means that the ECU has detected unusually high voltage coming from the TCR relay.
How Serious is the Code?
If the TCM detects unusually high-voltage on the sensing circuit between it and the transmission control relay, it will put the transmission into limp mode to prevent damage to the internal transmission parts. So given the fact that the transmission will be locked in a single forward gear (usually 2nd or 3rd), a P0891 diagnostic trouble code is pretty serious.
Symptoms of a P0891 include:
- Limp Mode
- Transmission can’t shift
- Wiring or connector damage
- Failure of the transmission control relay
- ECU or TCM programming problems
How to Diagnose the Code?
The first thing that should be done when trying to diagnose the cause of a P0891 OBDII trouble code, is to check the battery voltage output and the battery terminals. If the battery terminals are good then the transmission control relay will need to be tested. If everything checks out up to this point, then the wiring and connectors on this circuit, will need to be checked for damage, corrosion and shorts. The fuses and fusible links may also need to be checked.
What Repairs Will Fix P0891?
- Repair or replace damaged wiring and/or connectors
- Replace transmission control relay
- Update ECU or TCM software