Default or "Limp Home" Mode

If the ECU receives an illogical signal (usually indicating an open circuit, short circuit, or defective part), it will automatically switch to a fixed operating mode. The engine will continue to run, but none of the self adjusting devices will operate. This will cause the engine to run less efficiently, and may make it difficult or impossible to restart cold.
Upon switching to the default mode of operation, the ECU will emit a fixed "on-off ratio" signal that can be read with the proper test equipment (Bosch KDJE P600, or equivalent). The fault codes take priority over the normal ECU functions, so the system will not operate correctly until all problems are corrected.