DTC 13

Code 13 is set when the voltage at terminal D7 of the ECM:
^ Stays within a range of 0.35 to 0.55 volts;
^ The TPS is above a specified value;
and
^ The engine has run a specified time after start.

The ECM supplies a voltage of about 0.45 volt between terminals D6" and "D7".(If measured with a 10 megohm digital voltmeter, this may read as low as 0.32 volts.) The O2 sensor varies the voltage within a range of about 1 volt if the exhaust is rich, down through about 0.10 volt if exhaust is lean.

The sensor is like an open circuit and produces no voltage when it is below 360°C (600°F). An open sensor circuit or cold sensor causes open loop operation.
1. Grounding the diagnostic terminal with the engine running enables the "Field Service Mode ", which allows the ECM to confirm either open or closed loop operation using the "CHECK ENGINE" light.
2. This step verifies no additional codes stored, and that stored Code 13 is intermittent. See Symptoms. - Intermittent Malfunctions
3. This step simulates a lean exhaust. If the ECM and wiring are OK the ECM will see the lean condition and turn the "CHECK ENGINE" light off for at least 15 seconds after engine start.