Field Service Mode
In this mode, the SES light will indicate if the ECM is operating in "open loop" or "closed loop". "Open loop" operation is indicated when the SES light flashes steadily at 2.5 times per second. "Open loop" operation exists when the O2 sensor signal is not being processed by the ECM. "Closed loop" operation is indicated by a SES light that flashes once per second. The SES light will stay "off" most of the time (indicating a lean condition) or "on" most of the time (indicating a rich condition). "Closed" loop operation exists when the ECM is receiving and processing the O2 sensor signal.
When the system is in "field service mode", new fault codes cannot be stored in the ECM memory. When the vehicle is in "field service mode", it will go into "closed loop" as soon as the O2 sensor is at operating temperature and the internal timer which switches the vehicle to "closed loop" operation is by-passed.