Circuit Description

DATA TROUBLE CODE: P0125

SYSTEM: Insufficient Coolant Temperature for Closed Loop Fuel Control





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 the 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. 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.

DIAGNOSTIC TROUBLE CODE DETECTING CONDITION
- After the engine is warmed up, heated oxygen sensor output does not indicate RICH even once when the following conditions continue for at least 2 minutes:
- Engine speed: 1,500 rpm or more.
- Vehicle speed: 40 km/h (25 mph) or more.

TROUBLE AREA
- Open or short in Heated Oxygen Sensor circuit.
- Heated Oxygen Sensor.