Open circuit, chafed wiring or poor connection in the MIL request line between the transmission control module and the engine control module; Corroded, loose or pushed-out pins in the transmission control module or engine control module connectors; Water ingress or damage to the transmission harness or its connector, particularly at the gearbox; Fault in the engine control module's receiving circuit or communication with the transmission control module; Low or unstable supply voltage or poor earth to the transmission control module; Internal fault in the transmission control module.

P0802

Transmission Control System MIL Request Circuit/Open.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
8: AreaTransmission
02: FaultThe specific fault in that area

What it means

P0802 is the generic code for a fault in the circuit the transmission control module uses to ask the engine control module to switch on the malfunction indicator lamp (MIL). The transmission control unit detected an open circuit or no valid response on this MIL request line. It usually points to a wiring, connector or communication problem rather than a mechanical gearbox fault.

Symptoms

Malfunction indicator lamp on, or the lamp failing to behave as expected
Other transmission fault codes stored alongside P0802
Gearbox warning light or limp-home mode on some vehicles
Harsh, delayed or erratic gear changes if the transmission has also gone into a protective strategy
No obvious symptoms other than the warning lamp

Possible causes

Open circuit, chafed wiring or poor connection in the MIL request line between the transmission control module and the engine control modulelikely
Corroded, loose or pushed-out pins in the transmission control module or engine control module connectorslikely
Water ingress or damage to the transmission harness or its connector, particularly at the gearboxpossible
Fault in the engine control module's receiving circuit or communication with the transmission control modulepossible
Low or unstable supply voltage or poor earth to the transmission control modulepossible
Internal fault in the transmission control modulerare

How to diagnose

Read all stored codes and freeze frame data, noting any other transmission, network or voltage-related codes and whether P0802 is current or historic
Check the battery condition and charging voltage, as low voltage can cause spurious circuit faults
Visually inspect the transmission and engine control module wiring and connectors for damage, rubbing, corrosion, moisture or bent pins
Check the power supplies and earths to the transmission control module against the manufacturer's wiring information
Consult the model-specific wiring diagram to identify the MIL request circuit, as its routing and signal type vary by make and model
With the circuit isolated, test continuity and resistance to earth and supply on the MIL request wire
Clear the code, then wiggle-test the harness and connectors while monitoring live data to see if the fault returns
Check for applicable manufacturer software updates or technical bulletins before condemning a control module

How to fix

Repair or replace damaged wiring in the MIL request circuit
Clean, repair or replace corroded or damaged connectors and terminals
Correct any power supply or earth fault to the transmission control module
Repair the cause of water ingress and replace the affected harness section or connector
Replace and, where required, program the faulty control module only after wiring and supply faults are ruled out

Can I keep driving?

Medium severity

The car can usually be driven short-term if gearbox operation feels normal, but get it checked soon as the lamp may mask other faults and the transmission may enter limp mode. If shifting becomes harsh, erratic or stuck in a gear, limit driving and have it inspected promptly.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.