OBD-II trouble code
P0891: TCM Power Relay Sense Circuit High
The relay feedback line is reporting voltage at a moment when there should be none. The classic cause is contacts that have welded shut — and a welded relay is usually the consequence of another fault rather than the fault itself.
Quick facts
- System
- Powertrain
- Category
- Transmission / Electrical Supply
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $90 – $1,600
- DIY difficulty
- Advanced DIY
What does P0891 mean?
This is the one code in the transmission control module supply group that is set by something not happening. Every other member describes a circuit that failed to do what it was asked. This one describes a circuit that did something it was never asked to do — the sense line reported voltage present while the module was not commanding the relay on — and that shifts the entire diagnostic approach, because the test that proves it is performed with nothing switched on at all.
Start with the threshold, because readers coming from the power input codes will bring the wrong number with them. On a supply input, "high" is a comparison against a charging-system limit somewhere around sixteen volts, and the sensible first suspicion is the alternator or its regulator. A sense line is not measured against anything of the kind. It is compared against a logic threshold — a fraction of the module's reference voltage — so a perfectly ordinary twelve or thirteen volts sets this code the instant it appears at the wrong moment. Nothing about the charging system is implicated. A vehicle can have a flawless alternator, a healthy battery and textbook system voltage and still store P0891 all day, and time spent load-testing the charging system is time spent answering a question this code did not ask.
With the threshold understood, the fault splits cleanly in two. Either the relay contacts are closed when they should be open, or something is feeding voltage into the sense wire from elsewhere. Contacts closed when they should be open means welded — the moving contact has fused to the fixed one and the relay cannot release, no matter what its coil is told. This is a genuinely mechanical failure inside a sealed component, and it is common enough on transmission control relays that it should be the first assumption rather than an exotic one. The alternative, voltage arriving from another circuit, is the familiar short to a supply: a chafed sense wire lying against a live conductor, a crushed section of harness, or a repair that put two wires into one splice.
The part that matters financially is what a welded relay means about the rest of the car, and it is routinely skipped. Contacts do not weld because a relay was old. They weld because they carried a current surge or arced heavily while switching under load, and the current that did it came from the circuit the relay feeds. On this relay that circuit is the solenoid supply. A shorted shift solenoid, a chafed solenoid feed touching ground, or a harness pinched inside the transmission pan can pull far more current than the contacts were designed to interrupt, and the contacts fuse the next time they try to open under it. Replace the relay, leave the short in place, and the new relay welds too — sometimes within days. So the honest sequence on this code is to find out why, not merely to find out what. Measuring the current draw of the solenoid circuit, and checking the internal harness where it passes the pan gasket, is the difference between a repair and a repeat.
There is a driving consequence to understand as well, and it is not the one people expect. A welded relay leaves the solenoid supply permanently live, which sounds alarming but does not by itself move the transmission — the solenoids still only act when the module drives them. What it does mean is that the module can no longer guarantee it can cut power to those solenoids if something goes wrong, and that guarantee is the whole reason the relay exists. Most designs respond by refusing to shift and holding a fixed ratio, which is why the car behaves much like it does on the other codes in this group despite the underlying fault being the opposite of theirs.
Common causes
- Welded transmission control relay contacts holding the output permanently live
- Shorted shift solenoid drawing enough current to fuse the relay contacts when they open
- Chafed solenoid feed inside the transmission harness pulling excessive current to ground
- Sense wire shorted to a battery or ignition supply elsewhere in the harness
- Crushed or pinched harness section bridging the sense wire to a live conductor
- Incorrect relay fitted with a current rating below the circuit's requirement
- Previous wiring repair splicing the sense wire into the wrong circuit
- Internal relay failure after a jump start or a heavy load event
- Corrosion bridging adjacent terminals inside the relay socket
- Control module sense input failing high, which is uncommon and diagnosed last
Symptoms
- Check engine light with P0891 stored, often accompanied by shift solenoid codes
- Transmission held in a single fixed gear with no shifting
- Relay audibly failing to release, or no click at all when the ignition is switched off
- Repeat relay failure, with a replacement part failing again within days or weeks
- Blown transmission or solenoid fuse, sometimes repeatedly
- Harsh or absent engagement when selecting drive or reverse
- Fault present continuously rather than intermittently, unlike most codes in this group
- Scan tool showing the relay circuit as active when it has not been commanded
- Burning smell from the relay or fuse box in severe cases
- Warning message referring to transmission service or reduced performance
Diagnostic steps
- 1.Ignore the charging system first. This code compares the sense line against a logic threshold, not against a charging limit, so ordinary system voltage sets it when it appears at the wrong moment.
- 2.Read the sense circuit state with the relay not commanded. Voltage present with no command is the signature of welded contacts.
- 3.Remove the relay from its socket and check whether the sense line goes dead. If it does, the relay was holding the circuit live.
- 4.If the sense line stays live with the socket empty, look for a short to a supply in the harness rather than a relay fault.
- 5.Measure the current draw of the solenoid supply circuit before fitting a new relay, since the current that welded the contacts came from that circuit.
- 6.Test each shift solenoid for resistance and for a short to ground, comparing against the specification for that transmission.
- 7.Inspect the internal transmission harness where it passes the pan gasket and the case connector, a known chafe and pinch location.
- 8.Check the fitted relay is the correct part number and current rating, since an under-rated substitute welds readily.
- 9.Inspect the relay socket for corrosion bridging adjacent terminals, which mimics a welded relay without one being present.
- 10.Consider the module sense input only after the relay, the solenoid circuit and the harness have all been proven good.
Repair cost
$90 – $1,600
The relay itself is the cheap part of this repair and often not the whole repair. Diagnosis is $110 to $280. A transmission control relay is $20 to $180 fitted, and if it is replaced without finding out why it welded it is very likely to be bought twice. Testing and repairing the solenoid supply circuit runs $150 to $600. Replacing a failed shift solenoid is $250 to $700 depending on whether it sits externally or requires the pan and valve body to come down; an internal harness or case connector replacement is $300 to $900 because the pan and fluid have to come out to reach it. Repairing a short to voltage elsewhere in the harness is $150 to $600. Control module replacement with programming is $600 to $1,000 and is rarely where this code ends.
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DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.