Description
DTC 21 Main Heated 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 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 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 output of abnormal electromotive force, the ECM is unable to perform accurate air-fuel ratio control.
DIAGNOSTIC TROUBLE CODE DETECTING CONDITION
Main oxygen sensor signal voltage is reduced to between 0.35 volts and 0.70 volts for 60 seconds under the following conditions:
- Engine coolant temperature: 80°C (176°F) or more.
- Engine speed : 1,500 rpm or more.
- Load driving (Ex. A/T in overdrive (5th for M/T), A/C ON, Flat road, 50 mph).
- Main oxygen sensor signal voltage alternating above and below 0.45 volts.
TROUBLE AREA
- Main oxygen sensor circuit.
- Main oxygen sensor.
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.
1. Disconnect the EFI fuse (15 A) for 10 seconds or more, with ignition switch OFF.
2. Initiate test mode (Connect terminal TE2 and E1 of Data Link Connector 1 or 2 with ignition switch OFF).
3. Start the engine and warm up with all accessory switches OFF.
4. After the engine is warmed up, let it idle for 3 minutes
5. After performing the idling in step 3, perform gradual acceleration within the range 1,300 to 1,700 rpm (centered around 1,500 rpm) with the A/C switch ON and "D" position for A/T (5th for M/T).
NOTE: 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 40 - 50 mph.
7. Keep the vehicle running for 1 - 2 minutes after starting acceleration.
- If a malfunction exists, the malfunction indicator Lamp will light up after approximately 60 seconds from the start of acceleration.
NOTE: If the conditions in this test are not strictly followed, detection of the malfunction will not be possible.