Nissan X-trail MIL ON

Vehicle model: Nissan X-trail

Year: Nov., 2009

Type of engine: QR25

VIN: TDBNLJWT31EWAADB

Fault phenomenon:

Repair man said that this vehicle (see Figure 1) is an accident car. The engine MIL (Malfunction Indicator Light) is always on after having it repaired, and black smoke is expelled from exhaust pipe.



Fault diagnosis:

The vehicle, electronically controlled, need to be diagnosed with the help of Fcar auto diagnostic tool.

1. First, connect OBD test head and the main unit with test lines respectively;

2. Find the location of diagnostic socket ( see Figure 2);

3.Connect diagnostic socket, and turn ignition switch to ON position;

4.Turn on the power of Fcar car diagnostic scanner,

Enter the menu and select [China]——[Nissan]——[v7.1]——[Automatic detection system]——[Read DTC] DTC displays [P0136 HO2S2 (B1) heated oxygen sensor 2 (bank 1), P0036 HO2S2 HTR (B1) heated oxygen sensor 2 heater (bank 2)] ——[Clear DTC]——[DTC is cleared successfully] {MIL on the instrument is ON all the time.} ——[Read DTC]——DTC displays [P0136 HO2S2 (B1) heated oxygen sensor 2 (bank 1), P0036 HO2S2 HTR (B1) heated oxygen sensor 2 heater (bank 2)]—— indicating that this DTC is a true DTC.

诊断座：Diagnostic socket

Fault Analysis

According to the analysis of DTCs, this is an abnormity caused by short circuit of O2 sensor and damage of ECU.

Inspection methods for oxygen sensor are as follows:

1.Check of resistance of O2 sensor heater: When the temperature of the engine is normal, remove the wire connector of oxygen sensor and detect the resistance value between terminals of pressure sensor with ohmmeter. The resistance value should meet the requirements of standard value of specific models (generally, 4-40Ω). If the resistance value does not meet the requirements, please replace the O2 sensor.

2.Detection of O2 sensor voltage output signal: After mounted the wire connector of O2 sensor, draw out a wire from signal terminal, start the engine, make the engine reach normal operating temperature and keep the engine running at idle. In this case, please detect the output voltage of O2 sensor signal voltage. When removing the high-pressure cutoff line of a certain cylinder, oxygen content in exhaust gas will decline. If the voltage indicated by voltmeter rises, it means that the performance of O2 sensor is ok (output voltage of O2 sensor is generally between 0.2 and 0.9V and its range of variation is about 0.5V. Problems to be paid attention to during testing: no short circuit is allowed, the positive, negative connector of sensor terminal must not be screw up, the negative end of voltmeter is connected to the negative end of battery and the positive end is connected to the signal line of the sensor.

3.When inspecting the O2 sensor, fault cause can be known through observing the color of the top of oxygen sensor sometimes. The normal color of the top of O2 sensor is light gray. Once the color changes, it indicates fault or hidden fault of O2 sensor. For example, a black top of O2 sensor was caused by carbon pollution. Please clear carbon deposit on it after having it removed. When the top of oxygen sensor is found to be red, it means that O2 sensor is polluted by lead, due to the leaded gasoline used by the vehicle. Data from studies show that the function of O2 sensor shall lose basically, so that the three-way catalytic converter is polluted, making the purification efficiency is greatly reduced or even not function any more, after the vehicle has been driven for about 500 miles with leaded gasoline. If the O2 sensor has a white top, it was caused by silicon pollution. This is because undesirable silicone sealant is used when repairing the engine. At this moment, the O2 sensor must be replaced. Silica gel is also called Room Temperature Vulcanization (RTV) glue. If silica gel containing acetic acid is used on the engine parts where lubricating oil flows, acetic acid will be evaporated into crankcase or valve areas, then passing exhaust gas recirculation system and entering intake air pipe. Under normal condition, it shall go by the engine and be exhausted from exhaust pipe, thus damaging the O2 sensor.

4.Check of the operating voltage of O2 sensor: turn ignition switch to OFF position, remove the plug of NO.2 sensor, turn ignition switch to ON position, and then measure whether the voltage of ECU, earth wire, and signal line is normal with multimeter. If abnormal, it indicates that the circuit is shorted and the ECU is damaged at the time when the vehicle is crashed. You need to remove the harness connector of ECU and O2 sensor. Test with multimeter whether the circuit is connected.       

5.Check the ECU under the circumstance of ensuring O2 sensor and the circuits of O2 sensor are normal.

Troubleshooting:

When checking the circuits, we found that there are 2 circuits are not smooth, and then we found the circuit of O2 sensor socket is shorted. (See Figure 3)

After the treatment to harness short-circuited, re-install the components removed, turn the ignition switch to ON position. Perform the command [Clear DTC]——[DTC is clear successfully]. MIL is OFF. And everything is OK at normal test. The trouble is solved.

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