P0899 Transmission Control System MIL Request Circuit High

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.

Likely causes

Symptoms

• MIL on
• Gearbox faults may not be reported
• Limp mode as well if the gearbox module has no power

Repair estimate

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

Related codes

References: law.cornell.edu, saabwisonline.com.