Est Signal

The DIS ignition module sends a reference signal to the ECM when engine is cranking. If the engine RPM is under 400, the DIS ignition module controls the ignition timing. If engine RPM is above 400, the ECM applies a 5 volt signal to the by-pass circuit to switch spark timing control to the ECM.

An open or grounded EST circuit will result in engine operating in an ignition timing back-up mode and the "SERVICE ENGINE SOON" light will illuminate. If the EST fault is present the next time the engine is started, code 42 will be set.