Failure Modes

When a DTC is present in the PSCM, the EPAS enters 1 of 2 modes of operation.

The EPAS system enters a reduced steering assist mode  to protect the internal components of the EPAS system when a concern is detected by the PSCM such as low or high battery voltage or over-temperature concerns not considered to be a critical safety concern. This reduced steering assist mode gives the steering a heavier than normal feel.

The EPAS system enters a manual steering mode  (no electrical steering assistance is provided) when a concern considered to be a critical safety concern is detected. In manual steering mode, the vehicle has mechanical steering operation only, which gives steering operation a heavy feel.

There are 3 failure strategies used by the PSCM for determining failure modes. The first strategy is limp home; when the PSCM detects a fault requiring the EPAS to switch to a failure mode, the PSCM calculates a "safe" level of torque assist and provides this assist until the ignition is cycled. If the fault remains during the next ignition cycle, the PSCM switches to manual steering mode  until the fault is repaired. If the fault is not present during the next ignition cycle, the PSCM resumes normal operation and steering assist.

The next strategy is limp aside; similar to the limp home strategy except the limp aside strategy can recover normal steering assist during the same ignition cycle if the fault is no longer present or the ignition is cycled. If the fault is still present, the PSCM switches to manual steering mode  until the fault is repaired.

The last strategy is ramp down; the PSCM uses all the various inputs to calculate a torque output command to provide steering assist, if the module cannot calculate this output, it uses the last known good torque command to provide steering assist until the PSCM completely removes assist and switches to manual steering mode  until the fault is repaired.