DTC 21

DTC 21 Main Heated Oxygen Sensor Circuit





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: 0.0 volts).
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: 1.0 volt).
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 an output of abnormal electromotive force, the ECM is unable to perform accurate air-fuel ratio control.

DIAGNOSTIC 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.

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

Malfunction: Main Oxygen Sensor Deterioration



1. Disconnect the "EFI" fuse (15A) for 10 sec. or more, with IG switched OFF.
2. Initiate test mode (Connect terminal TE2 and E1 of DLC1 with IG switched OFF).
3. Start the engine and warm it up with all accessories switched OFF.
4. Idle the engine for 3 min.
5. Accelerate gradually within the range 1,300 - 1,700 rpm (Centered around 1,500 rpm) in D position for A/T (5th for M/T) and with the A/C switch ON.
(Take care that the engine speed does not fall below 1,200 rpm when shifting. Gradually depress the accelerator pedal and keep it steady so that engine braking does not occur).
6. Maintain the vehicle speed at 64 - 80 km/h (40 - 50 mph).
- Maintain the vehicle speed for 1 - 2 minutes after starting acceleration.
HINT: If a malfunction exists, the Malfunction Indicator Lamp will light up after approx. 60 seconds after the start of acceleration.

WIRING DIAGRAM





OX SIGNAL WAVEFORM INSPECTION USING OSCILLOSCOPE
- With the engine racing (2,500 rpm) measure waveform between terminals OX1 and E1 of engine control module.




- The correct waveform is as shown, oscillating between approx. 0.1 volt and 0.9 volt.




- If the oxygen sensor has deteriorated, the amplitude of the voltage will be reduced as shown.


INSPECTION PROCEDURE