DTC 21

Code Chart 21:

Oxygen Sensor Circuit:

CIRCUIT DESCRIPTION:

When the O2 sensor reaches operating temperature, it has a varying voltage from about 0.1 volt (exhaust is lean) to about 1.0 volt (exhaust is rich). The ECM uses this varying voltage signal to make fuel correction to maintain the optimum air/fuel ratio. 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.

TEST DESCRIPTION: Numbers below refer To circled numbers on the diagnostic chart.

1. This checks for a poor connection or for a faulty ECM. Code 21 will set under the following conditions:

^ Engine at normal operating temperature.
^ At least 2 minutes have elapsed since engine start-up.
^ O2 signal voltage is steady between 1 volt or 0 volt.
^ All above conditions are met for about 60 seconds.

2. This test determines if the O2 sensor is the problem or if the ECM and wiring are at fault.

DIAGNOSTIC AIDS:

Normal O2 sensor voltage varies between 0 mV to 999 mV (0.1 and 1.0 volt) while in a normal operating temperature. Code 21 sets in one minute if sensor signal voltage remains between 0 volt or 1 volt. Verify a clean, tight ground connection for BROWN wire. Open BLACK wire or BROWN wire will result in a Code 21. If Code 21 is intermittent, Testing and Inspection