OBD-II trouble code
P0886: TCM Power Relay Control Circuit Low
A control wire shorted to ground holds the relay closed, which means the transmission controller stays awake with the key out. The complaint that brings this car in is usually a flat battery every morning — not a shifting problem at all.
Quick facts
- System
- Powertrain
- Category
- Transmission / Electrical Supply
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $900
- DIY difficulty
- Advanced DIY
What does P0886 mean?
The consequence of this particular fault is easy to miss, because it does not announce itself the way a transmission code normally would.
The relay that supplies the transmission control module is switched on by the control module grounding one end of its coil. That is a deliberate design: ground is the default "off" state only because the module chooses when to apply it. If the control wire is shorted to ground somewhere along its length, that decision is taken out of the module's hands. The coil now has a permanent ground path, the relay is energised whether or not anything wanted it to be, and the transmission control module is powered continuously — including with the ignition off, the key in your pocket and the car locked in the driveway.
A control module left awake overnight draws current constantly. That current is far more than the vehicle's normal sleeping draw, and the result is a battery that is noticeably down by morning and flat within a couple of days of standing. So the complaint that arrives at the workshop is frequently not about the transmission at all. It is "the battery keeps going flat", or "it needs a jump start every Monday", and the owner has often already bought one or two batteries trying to solve it. Anyone hunting a parasitic drain by pulling fuses one at a time will eventually find the transmission circuit is the one responsible — and this code, if it has been read, explains why before the fuse-pulling starts. A relay that is still warm to the touch an hour after the car was parked is the same evidence available without any tools at all.
The second point concerns how the circuit must be tested, because the module's own self-protection interferes with the obvious approach. The low-side driver that grounds the relay coil is a semiconductor with current sensing built in. When it detects a short to ground it shuts itself off to avoid destroying itself, and on many vehicles it stays latched off until the next ignition cycle. That means live measurements taken while the module is protecting itself can look entirely normal, and it means repeatedly commanding the relay with a scan tool may give you a different answer each attempt depending on whether the driver has reset.
The way around that is to take the module out of the equation. Disconnect the relay and disconnect the module connector, then measure resistance from the control wire to a good chassis ground with nothing energising it. A reading near zero with both ends isolated is unambiguous — the wire itself is grounded somewhere along its route, and the module and relay are both innocent. A normal high reading with everything disconnected means the wire is fine, which leaves the relay coil and the module's driver as the remaining candidates, and those can then be separated by reconnecting one at a time. This is a static test that gives the same answer every time, which is exactly what is needed on a fault the module keeps hiding.
Finally, a short to ground does not appear anywhere at random. It happens where a wire meets metal, and the route this particular wire takes is predictable: it runs from the control module to the fuse box or relay centre, which means it crosses at least one bulkhead grommet and is bundled along body seams and brackets for most of its length. Grommets harden and split with age and let the harness bear on a sheet metal edge. Brackets and clips work loose and allow the loom to rub with engine movement. Previous accident repairs, aftermarket alarm or stereo installations and anything screwed or riveted through a panel are all places where a wire has been pierced or pinched. Inspecting those specific locations first is far quicker than tracing the whole run, and it is where this fault is genuinely found.
Common causes
- Relay control wire chafed through to bare metal at a bulkhead grommet
- Harness rubbing on a body seam, bracket or sheet metal edge where a clip has come loose
- Wire pierced or pinched by an aftermarket alarm, immobiliser or stereo installation
- Damage from previous accident repair or bodywork where the loom was disturbed
- Water ingress into the fuse box or relay centre creating a conductive path to ground
- Corroded relay socket bridging the control terminal to an adjacent ground terminal
- Internally shorted relay coil grounding the control side
- Crushed or trapped harness where a panel, bracket or fastener has been refitted over it
- Rodent damage to the harness, which characteristically produces shorts rather than opens
- Failed low-side driver inside the control module holding the output grounded, which is the last conclusion rather than the first
Symptoms
- Battery flat or badly discharged after the vehicle stands overnight or over a weekend
- Repeated jump starts required, often with one or more batteries already replaced
- Transmission control relay warm to the touch long after the ignition has been switched off
- Relay audibly clicking on as soon as the battery is connected, before the key is turned
- Check engine light with P0886 stored, sometimes alongside low-voltage codes from other modules
- Transmission entering fail-safe or holding a fixed gear once battery voltage falls
- Measurable parasitic current draw far above the vehicle's specification
- Fault appearing shortly after bodywork, an aftermarket electrical installation or harness disturbance
- Blown fuse in the transmission control circuit in cases where the short is heavy enough
- Erratic shifting appearing only as the battery weakens, masking the actual electrical cause
Diagnostic steps
- 1.Ask whether the battery has been going flat. This code frequently arrives as a parasitic drain complaint rather than a transmission complaint.
- 2.Feel the transmission control relay after the vehicle has been parked for an hour. A relay still warm with the key out is holding closed and confirms the fault without any equipment.
- 3.Measure parasitic current draw with the modules asleep and compare it with the manufacturer's specification.
- 4.Disconnect both the relay and the control module connector before testing, since the module's driver protects itself against a short and can make live readings look normal.
- 5.With both ends isolated, measure resistance from the relay control wire to chassis ground. A near-zero reading convicts the wire itself.
- 6.If the isolated wire measures normally, reconnect the relay alone and retest to separate a shorted relay coil from a module fault.
- 7.Inspect the harness at every bulkhead grommet on its route, which is where hardened rubber lets a wire bear on a metal edge.
- 8.Follow the loom along body seams and brackets looking for loose or missing clips that have allowed rubbing with engine movement.
- 9.Ask about aftermarket alarms, immobilisers, stereos, accident repairs and any recent bodywork, and inspect those areas specifically.
- 10.After repair, confirm the relay drops out with the ignition off and recheck parasitic draw rather than relying on the code staying clear.
Repair cost
$100 – $900
Most of the money on this code goes on finding the short rather than on fixing it. Diagnosis including a parasitic draw test is $130 to $350. Repairing a chafed wire at a grommet or a rub point, once located, is $120 to $450 depending on access and how much of the loom has to be opened up. A relay is $15 to $180 fitted, and a corroded relay socket or fuse box repair is $150 to $500. If the battery has been deeply discharged repeatedly it will likely need replacing regardless of the wiring repair, at $150 to $400 — worth budgeting for, because a battery that has been flattened several times rarely recovers its capacity. Replacing the control module is $600 to $1,100 with programming and is genuinely rare on this fault, since a shorted harness explains the symptoms completely and the module's driver usually survives by shutting itself down.
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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.