Code 13 - Open Oxygen (O2) Sensor Circuit: Notes

The ECM supplies voltage of about .45 volt between terminals D7 and D6. If measured with a 10-megohms digital voltmeter, this may read as low as .32 volts. The oxygen sensor varies the voltage within a range of about one volt (rich exhaust) to .1 volt (lean exhaust). The oxygen sensor is like an open circuit and produces no voltage when it is below about 600°F (316°C). An open sensor circuit or cold sensor causes open loop operation.

The 4.3L engine is equipped with an electrically heated oxygen sensor. The sensor is energized by the fuel pump relay, and remains on as long as the engine is running. Failure of the heating element, battery feed or ground circuits may cause Code 13 to be set at idle or low RPM.

1. Code 13 will set if the following conditions occur:
   • Engine at normal operating temperature.
   • At least 2 minutes engine running time after start.
   • Oxygen signal voltage steady between .35 and .55 volts.
   • RPM above 1600.
   • Throttle position sensor signal above 5% (about .3 volts above closed throttle).
   • All conditions must be met for about 60 seconds. If the conditions for a Code 13 exists, the system will not go into "closed loop".
2. This will determine if the sensor is at fault, wiring or ECM is the cause of the Code 13.
3. In doing this test, use only a high impedance digital volt/ohmmeter. This test checks the continuity of circuits No. 412 and 413. If circuit No. 413 is open, the ECM voltage on circuit No. 412 will be over .6-volts.