- Description of On-Board Diagnostics

Fig. 8 Oxygen sensor-indicator lamp:

Fig. 9 Feedback circuit cancel switch:

The operation of the oxygen sensor and ECM can be checked by using the sensor indicator lamp in the instrument cluster, Fig. 8. The sensor lamp flashes automatically at 30,000 mile intervals when the engine is at normal operating temperature.
The system check can be performed at any mileage if any of the following conditions occur:
a. Excessive fuel consumption.
b. Engine tends to stall.
c. Engine is hard to start.
Check system as follows:
1. Turn on cancel switch located in fuse box, Fig. 9.
2. Turn on ignition without starting engine. SENSOR lamp should light, but not flash. If lamp does not light, check wiring and bulb.
3. If lamp lights, start engine and allow to reach normal operating temperature.
4. With engine warmed up, increase speed to 1500-2000 RPM. Check that SENSOR lamp flashes to indicate system is functioning properly. If lamp does not flash, check for the following:
a. Defective oxygen sensor.
b. Defective mixture control solenoid valve.
c. Defective carburetor or improperly adjusted idle mixture.
d. Defective thermal switch.
e. Loose or disconnected emission control system wiring.
f. Defective ECM.
g. Defective idle or wide open throttle micro-switches.
5. After checking that SENSOR lamp flashes, shut off cancel switch in fuse box.
6. Shut off engine.