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by Bob O’Neill and Bob Lincoln
When the engine is starts cold, it runs rich (using more than an ideal amount of fuel) until the coolant temperature sensor reports a certain temperature — 86.9° F for the 1984 model year, 80.6° F for Turbo 2 models, and 95° for all other models. Once this happens the ECU waits 7.39 seconds, and then enters closed loop mode (where it uses feedback from the oxygen sensor and other sensors to meter fuel and air, rather than using built in tables). Then it waits another 59 seconds for the 1984 model year, or 11 minutes and 58 seconds (718 seconds) for later models. At this point it starts to monitor the signal from the O2 sensor sensor in the normal range.
If the O2 sensor output remains in the normal range for more than 21 seconds as sensed by the ECU a code 21 is set and the system remains in the closed loop mode. The engine is allowed to run rich and the ECU doesn’t adjust the air/fuel mixture to lean it out, but the power limited light is not lit.
Poor wiring or connections – Read the ‘How to troubleshoot drivability issues’. Connections from the O2 sensor and the ECU can cause code 21. Also a bad ground can contribute to the ECU not receiving the proper values from the O2 sensor.
Defective oxygen sensor – If the wiring is found to be good and Code 21 is still present then the O2 sensor should be replaced. The O2 sensor is located on the swing valve on turbo engines and on the exhaust manifold for non turbo engines. In both cases it’ll be found near the mounting flange for the down pipe. (Later cars, such as the Neon, used two oxygen sensors — one to test the results of the other.)
Sensors, Switches, and Other Systems | Main Repairs Page | EEKs
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