P0802: Transmission Control System MIL Request Circuit/Open

On older vehicles with a separate transmission module and no MIL message on the CAN bus, the transmission module signals 'MIL on' over its own wire to the engine module. P0802 checks this wire for an open circuit. GM's Duramax tables of 2004 and 2005 (Allison 1000) describe it: at key-on the transmission module briefly pulls the line active like a bulb check; if the engine module sees no off-to-on transition between 0.125 and 7 s after ignition, the line is treated as open – type B, MIL after two trips. Only then is P0700 evaluated. The wire itself must be monitored because an open line would stop transmission emission faults from lighting the MIL. Newer vehicles send the request over CAN, where faults appear as U codes.

Medium severity. Can I keep driving?Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.

Symptoms

Check engine lamp on
Transmission faults no longer light the MIL
Usually no change in driving

Most likely causes

MIL request wire or connector between the modules open or corroded
Transmission module without power or ground (fuse, relay)
Module output or input faulty
Wrong or retrofitted transmission module without this line

What to check, in order

Start with the simplest. You can tick off each step.

Check the fluid level and condition
Check the continuity of the wire between the component and the control module.
Read the input and output speed in live data
Check the connector and wiring
Check for other related engine codes
Consult the manufacturer's procedure for the transmission

Good to know

Repair difficulty: medium
It affects emissions.

What each character of P0802 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
8: Transmission
02: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0802?

Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.

What causes code P0802?

The most common causes are: mIL request wire or connector between the modules open or corroded; transmission module without power or ground (fuse, relay); module output or input faulty.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.