Description
DTC 26 Air-Fuel Ratio Rich MalfunctionWIRING 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
DTC 25
Main heated oxygen sensor voltage is 0.45 volts or less (lean) for 90 seconds under all of the conditions listed below:
(2 trip detection logic).
- Engine coolant temperature 70°C (158°F) or more.
- Engine speed 1,500 rpm or more.
DTC 25 OR 26
Engine speed varies by more than 20 rpm over the preceding crank angle period during a period of 25 seconds or more under all of the conditions listed below:
(2 trip detection logic).
- Engine speed: below 950 rpm
- Engine coolant temperature 80°C (176°F) or more
TROUBLE AREA
DTC 25
- Open or short in main heated oxygen sensor circuit
- Main heated oxygen sensor
- Ignition system
- ECM
DTC 25 OR 26
- Open and short in injector circuit
- Fuel line pressure (injector leak, blockage)
- Mechanical system malfunction (skipping teeth of timing belt)
- Ignition system
- Compression pressure (foreign object caught in valve)
- Mass air flow meter (air intake).
- ECM
DIAGNOSTIC TROUBLE CODE DETECTION DRIVING PATTERN
Purpose of the driving patterns
- 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.
Detection Driving Pattern For Open Or Short In Main Heated Oxygen Sensor
Detection Driving Pattern - Open or Short in Injector Circuit, Injector Leak or Blockage
