OBD-II trouble code
P0885: TCM Power Relay Control Circuit/Open
Two entirely separate circuits share the word "power" here, and this code is about the small one. It is the coil control wire that has gone open — not the heavy feed the relay switches — and testing the wrong one of the two is why this code has a reputation for being difficult.
Quick facts
- System
- Powertrain
- Category
- Transmission / Electrical Supply
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $80 – $1,100
- DIY difficulty
- Advanced DIY
What does P0885 mean?
Almost all of the confusion around this code comes from a single architectural point that is rarely explained, so it is worth setting out before anything else.
The transmission control module is not wired straight to the battery. Instead, a relay sits between the two, and that relay is what actually delivers current to the controller when the vehicle is running. This arrangement exists for good reasons: it lets the vehicle isolate the module completely when the key is off, it keeps the heavy current path short and thick rather than routing it through the ignition switch, and it lets the vehicle keep the module powered for a few seconds after shutdown so it can write what it has learned to permanent memory before losing supply.
A relay has two circuits inside it, and they have nothing electrically to do with each other. The heavy one — the contacts — carries the actual supply current to the module. The light one — the coil — is a small electromagnet that pulls those contacts closed when it is energised. Something has to decide when to energise that coil, and on most vehicles that something is the engine control module, which grounds one end of the coil to switch the relay on.
P0885 is entirely about the second circuit. It says the control module tried to operate the coil and detected an open — a break in the path it uses to energise the relay. It says nothing whatsoever about the heavy contact side, which may be in perfect condition. Anyone who reaches for this code and starts checking whether battery voltage arrives at the module's power pin is testing a different circuit from the one the code describes, and will draw a confident conclusion from an irrelevant measurement. Establishing which of the two circuits you are working on, before putting a probe anywhere, is most of the job.
On most vehicles the control module switches the coil's ground side rather than its supply side. That detail makes the diagnosis considerably simpler than it first looks, because it splits the circuit at a natural point. One end of the coil has permanent or ignition-switched battery voltage on it, delivered through a fuse. The other end runs to the control module, which pulls it to ground when it wants the relay on. So there are two checks and they answer different halves of the question. First, with the relay removed, confirm that voltage is present at the coil supply terminal in the socket — if it is not, the fault is upstream in the fuse or feed and the control side is innocent. Second, with the relay back in and a scan tool commanding it on and off, watch the voltage on the control terminal: it should sit near battery voltage with the relay off and fall close to zero when commanded on. If it stays high through the command, the module is trying and the path is broken, which is exactly what the code describes. If it never rises at all, the coil feed is missing. Between them those two measurements localise the fault to one side of the relay in about two minutes, and they can be done without a wiring diagram in hand.
One practical point about relays themselves. Most vehicles carry several identical ones in the same fuse box, and swapping a suspect relay for a known-good one from a non-critical circuit is free, takes seconds and is a perfectly legitimate first move. But calibrate what a negative result means: this code describes the control circuit, not the relay, so a swap that changes nothing has not wasted your time — it has removed the cheapest suspect and confirmed the code is pointing where it says it is. That is a useful outcome, not a dead end.
Common causes
- Open circuit in the wire between the control module and the relay coil
- Corroded, spread or backed-out terminal in the relay socket
- Blown fuse or broken feed supplying the relay coil
- Failed relay coil with an internally open winding
- Corroded or damaged connector at the control module carrying the coil control pin
- Chafed or broken wire where the harness passes through a bulkhead grommet or over a body seam
- Poor connection at a harness splice within the relay control circuit
- Water ingress into the fuse box or relay centre, corroding socket terminals
- Failed low-side driver inside the control module, which should be proven rather than assumed
- Incorrect or incompatible replacement relay fitted with a coil the module cannot drive
Symptoms
- Check engine light with P0885 stored, often with additional transmission codes
- Transmission stuck in a single default gear with no shifting at all
- No forward or reverse engagement in some cases, or engagement only in a limp-home ratio
- Scan tool unable to communicate with the transmission control module
- Relay not clicking when the ignition is switched on
- Transmission warning lamp or gear indicator illuminated or blank
- Speedometer or gear display behaving oddly where those signals pass through the transmission controller
- Vehicle drivable but with no torque converter lockup, producing high revs at cruising speed
- Fault present from the moment the key is turned rather than developing during driving
- Poor fuel economy as a consequence of running in a fixed gear without lockup
Diagnostic steps
- 1.Establish which circuit you are working on before probing anything. This code is about the relay's coil control circuit, not the heavy contact circuit that feeds the module.
- 2.Locate the transmission control relay using the vehicle's fuse box legend or wiring diagram, and identify which terminals are coil and which are contacts.
- 3.Swap the relay with an identical one from a non-critical circuit as a free first test, while recognising that a control circuit fault will not be cured by it.
- 4.With the relay removed, confirm battery voltage is present at the coil supply terminal in the socket. If it is absent, the fault is in the fuse or feed rather than the control side.
- 5.With the relay fitted, command it on and off with a scan tool while monitoring voltage at the coil control terminal.
- 6.Expect the control terminal to sit near battery voltage with the relay off and fall close to zero when commanded on. A terminal that stays high through the command confirms the open.
- 7.Check continuity of the control wire from the relay socket back to the module connector with both ends disconnected.
- 8.Inspect the relay socket terminals for corrosion, spreading and heat damage, and check their grip with a matching pin.
- 9.Inspect the harness where it passes through bulkhead grommets and over body seams, which is where an open in this circuit usually occurs.
- 10.Only consider the module's internal driver once the wiring, socket, fuse, feed and relay have all been proven good.
Repair cost
$80 – $1,100
The cheap outcomes are common on this code. A transmission control relay is $15 to $180 fitted, most of that being diagnostic time on a part that often costs less than lunch. A fuse or a repair to the coil feed is $80 to $220. Repairing a relay socket terminal, or replacing the socket, runs $120 to $400 depending on whether the fuse box can be serviced or has to be replaced as an assembly. Tracing and repairing an open in the control wire is $180 to $600, largely dictated by where the break sits and how much trim and harness has to come apart to reach it. Replacing the control module that drives the relay, with programming, is $600 to $1,100 — and on this code that should follow a proven-good wiring result rather than an assumption, because an open in the harness produces identical symptoms at a fraction of the price.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with transmission control relay replacement preselected. Adjust labor rate and vehicle category to fit your situation.
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.