Federal Models

DATA TROUBLE CODE: 21 / 28

NOTE: If DTC 21 is output inspect / replace front oxygen Sensor.
If DTC 28 is output, inspect / replace rear oxygen sensor.

SYSTEM: 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 engine control module (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 output of abnormal electromotive force, the ECM is unable to perform accurate air-fuel ratio control.

DIAGNOSTIC TROUBLE CODE DETECTING CONDITION
- 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).
- Oxygen sensor signal voltage alternating above and below 0.45 volts

TROUBLE AREA
- Oxygen sensor circuit.
- Oxygen sensor.

DIAGNOSTIC TROUBLE CODE DETECTION DRIVING PATTERN

Driving Pattern For Detecting DTC 21 And DTC 28:

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.