P0891
UniversalThe Transmission Control Module (TCM) detects an excessively high voltage condition in the power supply sense circuit for the TCM power relay, indicating an electrical fault.
Medium SeverityThis code indicates the Transmission Control Module detects a higher than expected voltage signal from its power supply relay circuit. This typically means the relay supplying power to the TCM is not functioning correctly, which can lead to erratic transmission operation or the vehicle entering limp mode.
⚠️ Before You Begin (Safety First)
Always disconnect the battery before performing any electrical work or replacing components to prevent accidental short circuits or electrical shock. Allow the vehicle to cool down before working on any hot engine or transmission components.
Common Symptoms
- Check Engine Light illumination
- Transmission shifting issues (hard shifts, erratic shifts)
- Vehicle entering limp mode
- Transmission not engaging a gear
- No communication with the TCM
Common Causes
- Faulty TCM power relay
- Short to voltage in the TCM power relay control circuit
- Corroded or loose connections at the relay or TCM
- Damaged wiring in the TCM power supply circuit
- Internal fault within the TCM (less common)
Frequently reported on: General/Universal
Diagnostic Steps
- Check fuses: Locate and inspect all fuses related to the transmission and TCM. Replace any blown fuses.
- Inspect TCM Power Relay: Locate the TCM power relay (often in the underhood fuse box) and inspect it for any signs of damage or corrosion. You can try swapping it with a known good, identical relay from another non-critical system (like the horn relay) to test if the relay is faulty.
- Inspect Wiring and Connectors: Visually inspect the wiring harness and connectors leading to the TCM power relay and the TCM itself for any signs of damage, fraying, corrosion, or loose connections.
- Clear Code and Test Drive: After performing any of the above steps, clear the P0891 code using an OBD-II scanner and test drive the vehicle to see if the issue returns.
👨🔧 When to Call a Professional
If replacing the TCM power relay does not resolve the issue, or if you are uncomfortable diagnosing complex electrical circuits, it is best to consult a professional automotive technician. Further diagnosis will involve advanced electrical testing with a multimeter and potentially an oscilloscope.
Commonly Asked Questions
What does 'TCM Power Relay Sense Circuit High' mean?
It means the vehicle's computer is detecting an unusually high voltage signal on the circuit that monitors the power supply to the Transmission Control Module (TCM), suggesting an electrical problem with the relay or its wiring.
Can I drive with a P0891 code?
It is not recommended to drive for an extended period. This code indicates a potential issue with the transmission's control system, which could lead to unpredictable shifting, limp mode, or even transmission damage if ignored.
Is it expensive to fix P0891?
The cost can vary. If it's just a faulty TCM power relay, it's typically an inexpensive fix. If the problem lies in the wiring or, in rare cases, the TCM itself, it can be more costly due to diagnostic time and part expense.
Where is the TCM power relay located?
The TCM power relay is typically found in one of the vehicle's fuse boxes, often under the hood or sometimes inside the passenger compartment. Consult your vehicle's owner's manual or a service manual for its exact location.
How do I test the TCM power relay?
You can test the relay using a multimeter to check for continuity and resistance across its terminals, or by applying voltage to the coil to see if it clicks and provides continuity on the switch side. Swapping it with a known good, identical relay is also a quick test.