Federal Models
DATA TROUBLE CODE: 28SYSTEM: Oxygen Sensor Circuit
NOTE: Diagnostic Trouble Code "21" is for the front Main Sensor circuit. Diagnostic trouble code "28" is for the rear Main 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 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 Engine Control Module of the RICH condition (large electromotive force: 1.0 volt).
The Engine Control Module 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 Engine Control Module 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 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).
- Oxygen Sensor signal voltage alternating above and below 0.45 volts
TROUBLE AREA
- Oxygen Sensor circuit.
- Oxygen Sensor.
DIAGNOSTIC 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.
Driving Pattern
It is vital that this test routine is adhered to detect the malfunction:
1. Disconnect the EFI fuse (20 A) for 10 seconds or more, with Ignition Switch OFF. Initiate test mode (Connect terminal TE2 and E1 of Data Link Connector 1 or 2 with Ignition 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 minutes.
4. After performing the idling in 3, perform gradual acceleration with in 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.
HINT: To 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 40 - 50 mph.
6. Keep the vehicle running for 1 - 2 minutes after starting acceleration.
NOTE: If a malfunction exists, the Malfunction Indicator Lamp will light up after approx. 60 sec. from the start of acceleration. If the conditions in this test are not strictly followed, detection of the malfunction will not be possible.
Malfunction Indicator Lamp
Normal Mode On
Test Mode On