Fault description: When the working temperature of the engine reaches 65 ℃ or more, the idling operation is unstable, and the warning lamp on the instrument panel "SERVICEENGINESOON" (engine early maintenance) is flashing.
    Check the process: use Ford's self-diagnostic system to read the fault code, with a jumper short-connect single-hole socket and 6-hole socket 2 feet, read the fault code display 15. Check the fault code table for oxygen sensor failure. For further confirmation, the oxygen sensor detection procedure is as follows:
    (1) Resistance check of oxygen sensor Remove the oxygen sensor wire connector, with a resistance meter to detect the oxygen sensor terminal resistance, the resistance to meet the standard value.
    (2) oxygen sensor voltage output signal detection. Install the oxygen sensor wire connector, start the engine, so that the oxygen sensor to the operating temperature, and maintain its idle operation. Use the voltmeter to check the output voltage terminal of the oxygen sensor terminal with a voltage value of 4.5-5.OV. When the throttle is fully open, the output voltage is 4.5-5.0v. The engine part of the vacuum tube pulled out when the detection, oxygen sensor output voltage is still 4.5-5.0v. If the air filter is blocked, and then detect the oxygen sensor output voltage remains at 4.5-5.OV, the diagnosis is determined to be due to oxygen sensor failure.
    Failure Analysis: Ford Motor Company in the exhaust system with three catalytic converters to reduce CO, HC and NOx emissions. As the three-way catalytic converter in the ideal air-fuel ratio of 14.7 near the highest purification rate, it must control the engine work in the ideal air-fuel ratio is very narrow range. In order to accurately control the air-fuel ratio in 14.7, in the engine control system commonly used by the oxygen sensor composed of air-fuel ratio feedback control, that is closed-loop control. The oxygen manifold is installed in the exhaust manifold or exhaust manifold in front of the ternary catalytic converter. The function is to detect the oxygen content in the exhaust gas to determine the actual air-fuel ratio. The theoretical air-fuel ratio is thick or lean, Feedback the corresponding voltage signal. The ECU controls the increase or decrease of the fuel injection amount based on the air-fuel ratio signal fed back by the oxygen sensor. Oxygen sensor normal output voltage signal in the 0.2-0.9V, and when the throttle fully open, the oxygen sensor output voltage signal in the 4.5-5.0v, indicating that the oxygen sensor failure, while the supply of thin or thick mixed gas, the sensor output voltage signal Can not be changed accordingly, then further determine the oxygen sensor failure. Due to the failure of the oxygen sensor, the computer can not get the oxygen sensor feedback signal, it can not be amended fuel injection, so that the idle operation when the engine is too thick or too lean mixture, resulting in engine instability.
    Maintenance program: replace the new oxygen sensor to do the test, failure to eliminate. Finally, clear the memory stored in the fault code.