The MIL request circuit is a dedicated wire over which the gearbox module asks the engine module to switch on the MIL. Today that request travels over CAN and shows as P0700 in the engine module; older Subarus, also sold as Saab 9-2X, used a single serial wire with its own codes P0864 to P0866. P0899 means the request wire stays high: an open line held up by the engine module's pull-up, a short to voltage or, with active-low logic, a gearbox module without power. No maker documents the code; Subaru's equivalent is P0866, set after more than 3 s high with MIL after two trips.
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
- MIL on
- Gearbox faults may not be reported
- Limp mode as well if the gearbox module has no power
Most likely causes
- Wire open (pull-up in the engine module)
- Short to voltage
- Gearbox module without power or ground
- Gearbox module faulty
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P0899 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 8: Transmission
- 99: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0899?
Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
What causes code P0899?
The most common causes are: wire open (pull-up in the engine module); short to voltage; gearbox module without power or ground.
Related codes
- P0898Transmission Control System MIL Request Circuit Low
- P0700Transmission Control System (MIL Request)
- P0866TCM Communication Circuit High
- P0865TCM Communication Circuit Low
- P0864TCM Communication Circuit Range/Performance
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.