Code EO13, Open Oxygen Sensor Circuit: Notes

Code EO13 is set when: Engine has run for 40 seconds, coolant temperature is above 109°F (42.5°C), voltage at terminal D7 of the ECM stays within a range of 350-550 mv, TPS is 6% above idle (.9-.10 volts), or condition lasts for 60 seconds consecutively.

The ECM supplies a voltage of about .45 volt between terminals D6 and D7. (If measured with a 10 megaohm digital voltmeter, this may read as low as .32 volts.) The oxygen sensor varies the voltage within a range of about 1 volt if the exhaust is rich, down through about .10 volt if exhaust is lean.

The sensor is like an open circuit and produces no voltage when it is below 600°F (360°C). An open sensor circuit or cold sensor causes "open loop" operation.

1. Allowing engine to run under these conditions will give a normally operating oxygen sensor sufficient time to cause system to run "closed loop". If voltage at this point is holding steady at about 450 mv, then the fault is present and must be further analyzed.
2. Inducing a direct short into the oxygen sensor signal circuit will cause the display to read near zero if the ECM and circuitry are okay. This would indicate a faulty sensor. If display stays near 450 mv, then an open circuit exists or the ECM is at fault.