DTC 21
CODE - 21 MAIN OXYGEN SENSOR CIRCUITCircuit 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.
Detecting Condition
*1 Refer to Trouble Code Descriptions, Note *5.
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.
(1) Disconnect the EFI fuse (15 A) for 10 sec. or more, with IG switch OFF. Initiate test mode (Connect terminal TE2 and E1 of data link connector 1 with IG switch OFF).
(2) Start the engine and warm up with all ACC switch OFF.
(3) After the engine is warmed up, let it idle for 3 min.
(4) After performing the idling in step (3), perform gradual acceleration with in the range 1,300 ~ 1,700 rpm (centered around 1,500 rpm) with the A/C switch ON and D position for A/T (5th for M/T).
(Take care that the engine speed does not fall below 1,200 rpm when shifting. Gradually depress the accelerator pedal and kept it. Steady so that engine braking does not occur).
(5) Maintain the vehicle speed at 40 - 50 mph (64 - 80 km/h).
(6) Keep the vehicle running for 1 - 2 min. after starting acceleration.
NOTE: If a malfunction exists, the malfunction indicator lamp will light up after approx. 60 sec. from the start of acceleration.
CAUTION: If the conditions in this test are not strictly followed, detection of the malfunction will not be possible.
Inspection Using Oscilloscope
- With the engine racing (2,500 rpm) measure between terminals OX1 and E1 of engine control module.
NOTE: The correct waveform appears as shown in the illustration above, oscillating between approximately 0.1 V and 0.9 V.
If the oxygen sensor has deteriorated, the amplitude of the voltage will be reduced as shown.
Diagnostic Chart
Wiring Diagram