DTC 28
CIRCUIT DESCRIPTION
To obtain a high purification rate for the Co, HO 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 increase 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: 1 V).
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.
California specification vehicles only:
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.
DETECTING CONDITION AND TROUBLE AREA
HINT:
- Diagnostic trouble code "21" is for the (main heated*1) oxygen sensor (Fr) circuit.
- Diagnostic trouble code "28" is for the (main heated*1) oxygen sensor (Rr) circuit.
*1: Main heated oxygen sensor ONLY for California specification vehicles
*2: Refer to "Two-trip Detection Logic" in Trouble Code Charts.
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.
Malfunction: (Main Heated) Oxygen Sensor Deterioration
- It is vital that this test routine is adhered to detect the malfunction:
1. Remove EFI fuse (30 A) for 10 seconds or more, with ignition switched OFF. Initiate test mode (Connect terminal TE2 and E1 of data link connector 2 with ignition switched OFF).
2. Start engine and warm up with all accessories switched OFF.
3. Let engine idle for 3 minutes.
4. With A/T in D position (5th for M/T) and A/C switched ON, gradually accelerate with range 1,300 - 1,700 rpm (centered around 1,500 rpm).
HINT:
- Ensure engine speed 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 speed to drop at any time during the test.
5. Maintain vehicle speed at 64 - 80 km/h (40 - 50 mph).
6. Keep vehicle running for 1 - 2 minutes after starting acceleration.
HINT: If a malfunction exists, the malfunction indicator lamp will light up after approx. 60 seconds from the start of acceleration.
NOTICE: If the conditions in this test are not strictly followed, detection of the malfunction will not be possible.
*: Main heated oxygen sensor ONLY for California specification vehicles.
DIAGNOSTIC CHART (Except California specification vehicles)
DIAGNOSTIC CHART (California specification vehicles only)
WIRING DIAGRAM
INSPECTION PROCEDURE (California specification vehicles only)
Reference: INSPECTION USING OSCILLOSCOPE
- With engine racing (4,000 rpm) measure waveforms between terminals OX1, OX2 and E1 of engine control module.
HINT: The correct waveform appears as shown in the above image, oscillating between approx. 0.1 V and 0.9 V
- If the oxygen sensor has deteriorated, the amplitude of the voltage will be reduced as shown above.