Limp-Home Mode or emergency mode will either be initiated by the ECM or the TCM.
ECM (Engine Control Module)
If a sensor should fail, provided the fault is not serious, the ECM will continue to operate the engine in Limp-Home Mode (emergency mode) using a default value for the missing signal. The ECM ensures that continuing to operate the engine will not cause damage or effect safety,otherwise an Engine shut-off process will be carried out.
TCM (Transmission Control Module)
In order to ensure a safe driving state and to prevent damage to the automatic transmission, the TCM control module switches to limp-home mode in the event of critical faults. A DTC (Diagnostic Trouble Code) assigned to the fault is stored in memory. All solenoid and regulating valves are thus de-energized.
The net effect is:
- The last engaged gear remains engaged.
- The modulating pressure and shift pressures rise to the maximum levels.
- The torque converter lockup clutch is deactivated.
In order to preserve the operability of the vehicle to some extent, the hydraulic control can be used to engage 2nd gear or reverse using the following procedure:
- Stop the vehicle.
- Switch off engine.
- Move selector lever to P: Park.
- Wait at least 10 seconds.
- Start engine.
- Move selector lever to D: 2nd gear.
- Move selector lever to R: Reverse gear.
The limp-home function remains active until the DTC is rectified or the stored DTC is erased with the DRBIII tool. Sporadic faults can be reset via ignition OFF/ON.
"Sporadic faults can be reset via ignition OFF/ON." That may work on low level problems... until it doesn't. If the condition(s) which triggered the LHM goes away when the engine is stopped, then all will be well until the vehicle operating conditions are such that they again go awry. The OFF/ON doesn't correct or repair anything.
LHM is a fairly complex system. More detail than most are willing to study is provided here.