1、 Sensor body malfunction

1. Thermal sensor failure
   Exhaust temperature sensors typically use negative temperature coefficient (NTC) thermistors, whose resistance decreases with increasing temperature. If the thermistor element ages or is damaged, it will cause abnormal resistance values (such as resistance not decreasing but increasing when the temperature rises), making the sensor unable to accurately reflect the exhaust temperature.
   • influenceECU receiving incorrect signals may misjudge the temperature of the catalytic converter, leading to the failure of protection mechanisms and even causing the catalytic converter to burn out.
2. The temperature sensitive surface is dirty
   Sensors exposed to high temperature exhaust gas for a long time are prone to carbon or oil accumulation on the surface, which hinders temperature conduction and leads to low measurement values.
   • influenceECU may delay or trigger alarms incorrectly, increasing the risk of catalytic converter overheating.

2、 Line and connection faults

1. Internal circuit open circuit/short circuit
   • open circuitThe sensor signal cannot be transmitted to the ECU, resulting in abnormal low temperature display in the data stream (such as poor contact of the grounding wire). The ECU misjudges it as a cold vehicle state, increases the fuel injection amount, and causes the mixture to be too rich.
   • short circuitThe signal voltage is pulled low, the data stream displays abnormal high temperature, and the ECU reduces the fuel injection amount, resulting in a lean mixture.
   • influenceDifficulty starting, unstable idle, excessive exhaust emissions, and even engine stalling or inability to start.
2. Wire harness damage or poor contact
   The sensor wires may be damaged due to engine vibration, high temperature aging, or external friction, resulting in signal interruption or interference.
   • Typical CaseThe bent part of the C172R aircraft exhaust temperature sensor wire is prone to wear, and wrapping it with heat shrink tubing can extend its service life.
   • influenceThe signal is intermittent, and the ECU cannot stably control fuel injection and ignition, causing intermittent faults (such as fault code 2085 indicating circuit intermittent problems).

3、 System chain reaction failure

1. Catalytic converter protection failure
   The main function of the exhaust temperature sensor is to monitor the temperature of the catalytic converter and prevent it from overheating and damage. If the sensor fails:
   • High temperature not alarmedCatalytic converters may sinter due to sustained high temperatures, leading to increased exhaust back pressure and uneven engine operation.
   • Low temperature false alarmFrequent triggering of alarms may interfere with normal ECU control and reduce fuel economy.
2. Excessive exhaust emissions
   Sensor malfunction causes the ECU to be unable to accurately adjust the air-fuel ratio, resulting in an increase in the content of pollutants such as NOx and CO in the exhaust gas, triggering an emission control system alarm.
   • data supportWhen the exhaust temperature of some car models exceeds 900 ℃, the sensor resistance value needs to be reduced to below 0.43k Ω to light up the alarm light; If the sensor fails, the alarm light may not light up, but the exhaust has seriously exceeded the standard.
3. Decreased fuel economy
   Abnormal sensor signals can cause imbalance in ECU fuel injection control, and excessive or insufficient mixture can increase fuel consumption.
   • caseWhen the grounding of the intake temperature sensor is poor, the data flow shows abnormally low temperature, and the ECU increases the fuel injection amount, resulting in a 10% -20% increase in fuel consumption.

Fault diagnosis and prevention suggestions

1. diagnostic method
   • resistance testPlace the sensor in ice or hot water and measure whether the change in resistance value meets the standard (such as 0.5-3.4V at 20 ℃ and 0.2-1V at 60 ℃).
   • Data flow analysisRead ECU data through a diagnostic instrument and check if the exhaust temperature signal matches the actual operating conditions.
   • Visual inspectionCheck for any damage, corrosion, or looseness in the sensor probe, wiring harness, and connectors.
2. preventive measures
   • regular maintenanceClean the carbon on the surface of the sensor, check the fixing of the wiring harness to avoid friction damage.
   • Standard operationDo not cut the wires randomly when replacing the sensor to prevent affecting the accuracy of the thermocouple circuit resistance.
   • Use genuine accessoriesPoor quality sensors have poor thermal stability and are prone to premature failure.

Although the malfunction of the exhaust temperature sensor is minor, it may trigger a chain reaction that affects engine performance, emissions, and safety. Regular inspection and maintenance are key to avoiding malfunctions.