DTC 21

CIRCUIT DESCRIPTION
To obtain a high purification rate for the CO, HC and NOx components of the exhaust gas, a three-way catalytic converter is used, but for most efficient use of the three-way catalytic converter, the air-fuel ratio must be precisely controlled so that it is always close to the stoichiometric air-fuel ratio.
The oxygen sensor has the characteristic whereby its output voltage changes suddenly in the vicinity of the stoichiometric air-fuel ratio. This characteristic is used to detect the oxygen concentration in the exhaust gas and provide feedback to the computer for control of the air-fuel ratio.
When the air-fuel ratio becomes LEAN, the oxygen concentration in the exhaust increases and the oxygen sensor informs the ECM of the LEAN condition (small electromotive force: 0 V).
When the air-fuel ratio is RICHER than the stoichiometric air-fuel ratio the oxygen concentration in the exhaust gas is reduced and the oxygen sensor informs the ECM of the RICH condition (large electromotive force: 1V).
The ECM judges by the electromotive force from the oxygen sensor whether the air-fuel ratio is RICH or LEAN and controls the injection time accordingly. However, if malfunction of the oxygen sensor causes output of abnormal electromotive force, the ECM is unable to perform accurate air-fuel ratio control.
The main heated oxygen sensors include a heater which heats the Zirconia element. The heater is controlled by the ECM. When the intake air volume is low (the temperature of the exhaust gas is low) current flows to the heater to heat the sensor for accurate oxygen concentration detection.





DETECTING CONDITION AND TROUBLE AREA

HINT: Diagnostic trouble code 21 is for the left bank main heated oxygen sensor circuit. Diagnostic trouble code 28 is for the right bank main heated oxygen sensor circuit.





DIAGNOSIS TROUBLE CODE DETECTION DRIVING PATTERN

Purpose of the driving pattern.:
- To simulate diagnostic trouble code detecting condition after diagnostic trouble code is recorded.
- To check that the malfunction is corrected when the repair is completed confirming that diagnostic trouble code is no longer detected.

Malfunction: Main Heated Oxygen Sensor Deterioration:
1. Disconnect the EFI fuse (30 A) for 10 sec. or more, with IG switched OFF. Initiate test mode (Connect terminal TE2 and E1 of data link connector 2 with IG switched OFF).
2. Start the engine and warm up with all ACC switched OFF.
3. Let the engine idle for 3 minutes.
4. With A/T in D position and A/C switched ON, gradually accelerate within the range 1,300 - 1,700 rpm (centered around 1,500 rpm).(Take care that the engine speed does not fall below 1,200 rpm. Gradually depress the accelerator pedal and keep it steady so that engine braking does not occur).
5. Maintain the vehicle speed at 64 - 80 km/h (40 - 50 mph).
6. Keep the vehicle running for 1 - 2 min. after starting acceleration.

HINT: If a malfunction exists, the Malfunction Indicator Lamp will light up after approx. 60 sec. from the start of acceleration.

NOTICE: If the conditions in this test are not strictly followed, detection of the malfunction will not be possible.





DIAGNOSTIC CHART

HINT: If diagnostic trouble code "21" is output, check the left bank main oxygen sensor circuit. If diagnostic trouble code "28" is output, check the right bank main oxygen sensor circuit.





WIRING DIAGRAM











INSPECTION PROCEDURE

Reference: INSPECTION USING OSCILLOSCOPE:
- With the engine racing (4,000 rpm) measure waveform between terminals OXL1, OXR2 and E1 of engine control module.





HINT: The correct waveform appears as shown in the above image, oscillating between approx. 0.1 V and 0.9 V.





- If the oxygen sensor is deteriorated, the amplitude of the voltage is reduced as shown in the above image.