FIELD SERVICE MODE DIAGNOSTIC CHART

A new diagnostic chart has been developed for all vehicles equipped with 1.8L, 2.5L, 5.0L or 5.7L EFI engines. This chart utilizes a capability of the system to better diagnose problems.

Field Service Mode: Engine running, diagnostic terminal grounded. "Check engine" light is "off" when exhaust is lean and "on" when it is rich.

Open Loop: "Check engine" light flashes at a rate of two (2) times per second.

Closed Loop: "Check engine" light flashes at a rate of one (1) time per second.

Lean Exhaust: "Check engine" light is out all the time or most of the Indication time.

Rich Exhaust: "Check engine" light is on all the time or most of the time. Indication

*Open loop at idle only may be caused by a cold oxygen sensor after a period of idling. If this is the cause, running engine at 2000-3000 rpm for one (1) minute should warm it up and give closed loop operation at idle.

After any repair, clear codes and confirm "closed loop" operation.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.