Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Field Service Mode
In this mode, the CHECK ENGINE light will indicate if ECM is operating in open loop or closed loop.
Open loop operation is indicated when the CHECK ENGINE light flashes steadily at 2 1/2 times per second. Open loop operation exists when the O2 sensor signal is not being processed by ECM.
If CHECK ENGINE light flashes once per second, closed loop operation is indicated. CHECK ENGINE light will remain either off most of the time (indicating a lean condition), or on most of the time (indicating a rich condition). Closed loop operation exists when ECM is receiving and processing O2 sensor signal.
When system is in field service mode, new fault codes cannot be stored in ECM memory. When vehicle is in field service mode, it will go into closed loop when O2 sensor is at operating temperature and internal timer is by-passed. Internal timer switches vehicle to closed loop operation.
Use the Manual With the Right Hardware
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.