Description

DTC 21 Main Heated Oxygen Sensor Circuit

WIRING DIAGRAM





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 engine control module (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.

DIAGNOSTIC TROUBLE CODE DETECTING CONDITION
A.- Open or short in heater circuit of main heated oxygen sensor for 0.5 seconds or more.
OR
B.- Main heated oxygen sensor signal voltage is reduced to between 0.35 volts and 0.70 volts for 60 seconds under all of the conditions below:
(2 trip detection logic)
- Engine coolant temperature between 80°C (176°F) and 95°C (203°F).
- Engine speed 1,500 rpm or more.
- Load driving (A/C ON, flat road, 50 mph, A/T in Overdrive or M/T in 5th gear).
- Main heated oxygen sensor signal voltage alternating above and below 0.45 volts

TROUBLE AREA
A.- Open or short in heater circuit of main heated oxygen sensor.
- Main heated oxygen sensor heater.
ECM.
OR
B.- Main heated oxygen sensor circuit.
- Main heated oxygen sensor.

DIAGNOSTIC TROUBLE CODE DETECTION DRIVING PATTERN

Driving Pattern For Detecting DTC 21:

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.