OBD-II trouble code
P0887: TCM Power Relay Control Circuit High
A transmission controller that will not talk to a scan tool is usually assumed dead. With this code stored it very often is not — it is a module that was never switched on, because the circuit that should pull its relay closed cannot be pulled down.
Quick facts
- System
- Powertrain
- Category
- Transmission / Electrical Supply
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $90 – $1,100
- DIY difficulty
- Advanced DIY
What does P0887 mean?
This code and its low-side sibling produce opposite behaviour in the car, and knowing which behaviour you are looking at tells you which one you have before a probe touches anything.
The control module operates the transmission relay by pulling one end of its coil down to ground. When that circuit is shorted to a voltage source, or is otherwise held up so the module cannot pull it down, the coil never sees the potential difference it needs. The relay does not close, the contacts never make, and the transmission control module is never supplied at all. Where the low-side fault leaves the relay stuck permanently on, this one leaves it permanently off — and the practical results could hardly be more different.
The most valuable consequence is what happens at the diagnostic socket, and it is worth being emphatic about because of what it saves. A transmission control module with no supply cannot respond to a scan tool. It does not appear in the module list, it will not report codes, and it looks in every respect like a controller that has failed. That appearance is convincing, and it is the basis on which a great many transmission control modules are condemned, ordered, fitted and programmed. But a module that has no power is not broken; it is switched off. The distinguishing test is trivial once the possibility is on the table: with the ignition on, measure whether battery voltage is actually arriving at the module's supply pins. If it is not, the module's silence is fully explained and there is no evidence whatsoever that the module is faulty. This code arriving alongside a no-communication complaint is therefore one of the most useful pairings in transmission diagnosis, and it points at a relay circuit rather than at the most expensive part in the system.
The wiring fault behind it also has a characteristic worth exploiting. A wire does not short to voltage on its own — it has to make contact with something that is carrying voltage, and in practice that means another wire in the same bundle whose insulation has failed at the same point. The two circuits chafe together, usually where the loom is under strain or bears on something hard, and from then on they are connected. The useful consequence is that the fault is rarely alone. Whatever else runs through that section of harness is now tied to this circuit, so there is often a second unrelated code stored, or a symptom the owner mentions in passing and does not connect to the transmission — an exterior lamp behaving oddly, a heated element that stays on, a switch that no longer does anything. Ask about it. Two unrelated complaints that both trace back to the same length of loom localise the damage far faster than tracing one circuit end to end, and the repair fixes both.
One caution about test technique, because it can hide the very fault you are looking for. Backprobing the control terminal with the relay still fitted puts the coil winding in parallel with your meter, and a coil is a low-resistance path to whatever is on its other end. That can pull a weakly-driven short to voltage down far enough to read as normal, and the circuit then passes a test it should have failed. Remove the relay first, then check for voltage on the control terminal in the empty socket with the ignition on. That terminal should be dead — it is an output the module grounds, not a supply. Any voltage sitting on it with the relay out is the short, plainly visible and no longer masked.
Common causes
- Relay control wire shorted to a battery or ignition voltage source elsewhere in the harness
- Chafed loom where two circuits have worn together and are now connected
- Water ingress in a connector or fuse box bridging the control pin to a live terminal
- Corroded relay socket bridging the coil control terminal to the coil supply terminal
- Internally shorted relay coil connecting supply directly to the control side
- Damage from an aftermarket electrical installation tapped into the wrong circuit
- Open circuit in the control wire, which on some strategies is also reported as a high state
- Terminal pushed out of position in the module or relay connector, contacting a neighbouring pin
- Previous repair or splice made into the wrong wire during earlier harness work
- Failed low-side driver inside the control module unable to pull the circuit down, proven only after the wiring is cleared
Symptoms
- Transmission control module not communicating with a scan tool and absent from the module list
- Check engine light with P0887 stored, read from the engine controller rather than the transmission one
- Transmission stuck in a single default gear with no shifting whatsoever
- No torque converter lockup, giving high engine speed at cruising road speed
- Transmission relay not clicking when the ignition is switched on
- Gear position display blank, dashed or frozen
- Speedometer or road speed dependent functions affected where those signals route via the transmission controller
- A second, apparently unrelated electrical fault elsewhere on the vehicle
- Fault present from the first turn of the key rather than developing while driving
- Poor fuel economy and reduced performance as a result of running in one fixed ratio
Diagnostic steps
- 1.If the transmission controller will not communicate, check whether battery voltage is actually reaching its supply pins before concluding the module has failed.
- 2.Note which way the fault has gone. A relay stuck off points here; a relay stuck on and a flat battery points at the low-side fault instead.
- 3.Remove the relay before testing the control terminal, because backprobing with the relay fitted puts the coil in parallel with the meter and can mask a weak short to voltage.
- 4.With the relay out and the ignition on, check for voltage at the coil control terminal in the socket. That terminal is an output the module grounds and should be dead.
- 5.Confirm the coil supply terminal has voltage as expected, so that a missing feed is not mistaken for a control circuit fault.
- 6.Check continuity of the control wire from the relay socket to the module connector, and check it for isolation from every other circuit in the same bundle.
- 7.Read all modules and ask the owner about any other electrical oddity, however unrelated it sounds. A chafed bundle usually produces two complaints from one fault.
- 8.Inspect the harness where it is under strain or bears on something hard, since a short to voltage requires two wires to have worn together.
- 9.Inspect the relay socket and module connector for water ingress, corrosion and displaced terminals contacting a neighbouring pin.
- 10.Prove the wiring good before condemning the module, and after any repair confirm the relay energises and the transmission controller returns to the scan tool's module list.
Repair cost
$90 – $1,100
What this code most often saves is the expensive outcome rather than incurring one. Diagnosis is $120 to $300. A relay is $15 to $180 fitted and a corroded socket repair $150 to $500. Repairing a chafed loom where two circuits have shorted together is $180 to $650 depending on where the damage sits and how much of the harness must be opened, and that single repair frequently fixes a second unrelated fault at no extra cost. Cleaning water ingress from a connector or fuse box is $120 to $400. Replacing the transmission control module with programming is $600 to $1,200 — and the reason to be careful here is that a module with no supply is silent, absent from the scan tool and indistinguishable from a dead one, so this is a code where an expensive part gets fitted against a circuit fault more often than almost anywhere else in the system.
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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.