Federal Models

DATA TROUBLE CODE: 25

SYSTEM: Air-Fuel Ratio Lean Malfunction

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
A. Difference of air-fuel ratio feedback compensation value between front (No. 1 to 3 cylinders) and rear (No. 4 to 6 cylinders) is more than 15 percent for 20 seconds or more under all of the conditions listed below:
(2 trip detection logic)
- Engine speed 2,000 rpm or more.
- Engine coolant temperature between 60°C (140°F) and 95°C (203°F).
OR
B. Engine speed varies by more than 15 rpm over the preceding crank angle period during a period of 20 seconds or more under all of the conditions listed below:
(2 trip detection logic)
- Engine speed idling.
- Engine coolant temperature between 60°C (140°F) and 95°C (203°F).

TROUBLE AREA
- 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)
- Volume air flow meter (air intake)
- ECM
OR
- 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)
- Volume 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