OBD-II trouble code
P0802: Transmission Control System (MIL Request) Circuit/Open
The wire the transmission module uses to ask for the warning lamp is open. That inverts the usual logic: the absence of other transmission codes now proves nothing, because the channel for reporting them is broken.
Quick facts
- System
- Powertrain
- Category
- Transmission
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $1,500
- DIY difficulty
- Advanced DIY
What does P0802 mean?
On vehicles where the transmission has its own control module, that module needs a way to tell the engine module that something is wrong so the malfunction indicator lamp can be lit. On a number of designs — particularly those from the era before everything moved onto a shared data bus, and on some later vehicles that kept a hardwired backup — that request travels down a dedicated wire between the two modules. P0802 says that wire is open.
The important consequence is one that turns normal diagnostic reasoning on its head. Ordinarily, scanning a vehicle and finding no transmission codes is reassuring. With P0802 present it is not, because the path by which a transmission fault would have been announced is the very thing that is broken. The transmission module may be sitting on a genuine fault it has been trying and failing to report. So this code should never be dismissed as merely a wiring nuisance on the grounds that nothing else is stored — the correct move is to read the transmission module directly, on its own, rather than relying on what appears in the engine module's list.
It is worth being precise about how this differs from its neighbours, because three codes in this area sound alike and mean different things. P0700 is the umbrella: the transmission module has a fault and has successfully said so, and the detail is stored elsewhere. P0800 is the same arrangement for the transfer case module. P0802 is not about a fault in either transmission or transfer case — it is about the request line itself being open, so it reports a failure of the reporting mechanism. A vehicle can carry P0802 with a completely healthy transmission, and it can equally carry P0802 while the transmission is in trouble and unable to say so.
Where the open usually is depends on the architecture. On a hardwired implementation it is a physical conductor running between two modules that are often at opposite ends of the engine bay, passing through at least one bulkhead grommet and frequently through an intermediate connector, so chafe, a pulled pin, corrosion at a bulkhead connector, or damage from previous work are all realistic. On vehicles that make the request over the data bus instead, the same code becomes a communication fault, and the investigation shifts to bus integrity, module power and grounds, and whether the transmission module is awake and responding at all — a module that has lost power or ground cannot make the request either, and that presents identically.
Aftermarket work deserves a specific mention here, because this circuit is a common casualty of it. Remote starters, alarms, aftermarket stereos, trailer wiring and previous accident repairs all involve cutting into looms near where this wire runs, and a request line that has been spliced, pinched behind a dash panel, or left unreconnected after a module was swapped is a frequent finding. If the code appeared shortly after any of that work, start there rather than at the modules.
Common causes
- Open conductor in the dedicated request wire between the transmission control module and the engine control module
- Corroded or pushed-back pin at a bulkhead or intermediate connector in that circuit
- Wiring damaged during previous repair work, accident repair or aftermarket accessory installation
- Transmission control module without power or ground, so it cannot make the request at all
- Failed driver output in the transmission control module
- Failed input at the engine control module end of the circuit
- Data bus fault on vehicles where the request is sent as a message rather than over a dedicated wire
- Blown fuse feeding the transmission control module
- Connector left unseated after a module replacement or a transmission removal
- Chafed harness where the loom passes a bracket, a bulkhead grommet or the back of the engine
Symptoms
- Check engine light with no drivability complaint whatsoever
- No transmission codes stored, which in the presence of this code is not reassuring
- Transmission warning lamp or message that never illuminates even when the transmission is misbehaving
- Transmission module not responding to a scan tool, or missing from the module list
- Shifting faults that the driver can feel but which no lamp ever reported
- Fault appearing shortly after a remote starter, alarm, stereo or trailer wiring installation
- Code returning immediately after being cleared, with no other symptom
- Emissions test failure on readiness or lamp status rather than on tailpipe results
- Transmission entering failsafe with no warning lamp accompanying it
Diagnostic steps
- 1.Read the transmission control module directly rather than relying on the engine module's code list. The reporting path is broken, so anything the transmission module has found may not have propagated.
- 2.Confirm the transmission control module is present, powered and communicating. A module with no power or ground cannot make the lamp request and presents exactly like an open circuit.
- 3.Establish from the wiring diagram whether this vehicle uses a dedicated request wire or sends the request over the data bus, because the two lead to completely different tests.
- 4.On a hardwired circuit, check continuity end to end between the two module connectors with both disconnected, wiggling the loom at bulkhead grommets and brackets while watching the meter.
- 5.Inspect connector terminals at both modules for pins that have been pushed back out of the housing, which is common after any connector has been disturbed.
- 6.Check the fuses feeding the transmission control module under load, not by eye.
- 7.Look for evidence of aftermarket work — remote start, alarm, stereo, trailer wiring — and inspect any splices found in the vicinity of this circuit.
- 8.On a bus-based implementation, check bus integrity and confirm the transmission module appears and responds, before assuming the fault is in a single wire.
- 9.Measure voltage drop on the transmission module ground path under load rather than checking continuity alone.
- 10.After repair, verify the function rather than the absence of the code: confirm the lamp responds when the transmission module is commanded to request it, if the scan tool supports that test.
Repair cost
$100 – $1,500
The common outcome is a wiring repair, which runs $150 to $600 depending on how much of the loom has to be opened and whether a bulkhead connector is involved. A pushed-back or corroded pin repaired at a connector is $100 to $300. A blown fuse or an unseated connector found during diagnosis can be under $150 in total. If the transmission control module itself has failed or lost its power supply, expect $500 to $1,500 including programming. A data bus fault on a bus-based implementation is $200 to $900 depending on how many modules have to be isolated to find it. Budget $100 to $200 for diagnosis, and be aware that whatever the transmission module turns out to have been unable to report is a separate cost on top of this one.
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Open the Repair Cost Estimator with module communication diagnosis 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.