DTC 26

Data Trouble Code No. 25 - Air-Fuel Ratio Lean Malfunction

Detecting Condition

Main Heated Oxygen Sensor voltage is 0.45 volts or less (lean) for 90 seconds under the following conditions (2 trip detection);

- Engine coolant temperature 70°C (158°F) or more.
- Engine speed 1,500 rpm or more.
or
Main Heated Oxygen Sensor voltage is alternating above and below 0.45 volts at 5 times per second or more under the following conditions (2 trip detection);

- Engine speed idling.
- Engine coolant temperature between 60°C (140°F) and 95°C (203°F).
or
Difference of air/fuel ratio feedback compensation value between right and left is more than 10 percentage for 30 seconds or more under the following conditions (2 trip detection);

- Engine speed: 2,000 rpm or more.
- Engine coolant temperature between 60°C (140°F) and 95°C (203°F).

Trouble Area

- Open or short in Main Heated Oxygen Sensor circuit.
- Main Heated Oxygen Sensor.
- Ignition system.
- Engine Coolant Temperature Sensor.
or
- Open or short in Injector circuit.
- Fuel line pressure (leak or blockage).
- Volume Air Flow Meter.
- Foreign object caught in valve.
- Engine ground bolt loose.
or
- Fuel line pressure (leak or blockage).
- Mechanical system malfunction (timing belt skipped).
- Ignition system

Malfunction Indicator Lamp

Normal Mode On
Test Mode On

Code 25 And 26 description:

Circuit Description
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 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 engine control module of the RICH condition (large electromotive force: 1 V).

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.

The main heated oxygen sensors include a heater which heats the Zirconia element. The heater is controlled by the engine control module. 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.

HINT: Diagnostic trouble code 21 is for the left bank main heated oxygen sensor circuit. Diagnostic trouble code 28 is for the right bank main heated oxygen sonsor circuit.