DTC 21

DTC 21 Main Heated Oxygen Sensor Circuit

Oxygen Sensor Cut-Away:

Oxygen Sensor Output Voltage Chart:

CIRCUIT DESCRIPTION
To obtain a high purification rate for the CO, HO 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: 1 V).
The ECM judges by the electromotive force from the oxygen sensor whether the air-fuel ratio is RICH or LEAN and controls the injection duration 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.
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

Detection Driving Pattern:

DIAGNOSTIC TROUBLE CODE DETECTION DRIVING PATTERN

Purpose of the driving pattern.
(a) To simulate diagnostic trouble code detecting condition after diagnostic trouble code is recorded.
(b) 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

It is vital that this test routine is adhered to detect the malfunction:
(1) Disconnect the EFI No. 1 fuse (30 A) for 10 sec. or more, with IG switch OFF. Initiate test mode (Connect terminal TE2 and E1 of data link connector 2 with IG switch OFF).
(2) Start the engine and warm up with all ACC switch OFF.
(3) Idle the engine for 3 min.
(4) Accelerate gradually within the range 1,30O ~ 1,700 rpm (centered around 1,500 rpm) with the A/C switch ON and D position for A/T (5th for M/T).

HINT:
- Ensure engine rpm does NOT fall below 1200 rpm.
- Gradually depress the accelerator pedal at a suitable rate to comply with the test requirements on the above graph.
- Never allow engine rpm to drop at any time during the test.

(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.





WIRING DIAGRAM

Step 1:

Step 2:

Step 3:

INSPECTION PROCEDURE

Reference INSPECTION USING OSCILLOSCOPE
- With the engine racing (4,000 rpm) measure waveform between terminals OX1 and E1 of engine control module.

Reference Waveform:

HINT: The correct waveform is as shown oscillating between approx. 0.1 V and 0.9 V.

Defective Oxygen Sensor Waveform:

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