What does it mean?
The humidity sensor reports the water content of the intake air so that fuelling, spark and emission strategies can be corrected for air density and combustion speed. Here the circuit is intact and the sensor is answering, but the answer does not fit the rest of the picture. The module compares the reported humidity against ambient temperature, barometric pressure, intake air temperature and learned values, and stores the code when the figure sits outside the plausible window or refuses to move while conditions clearly change. A sensor that has become slow, coated or drifted will fail this plausibility test long before it fails electrically.
Symptoms
- MIL on with a humidity value that looks implausible for the current weather
- Mild power loss or duller throttle response as protective calibrations take over
- On diesels, altered EGR behaviour and sometimes more smoke on acceleration
- Occasional knock or pinging under load on gasoline engines
- Slightly higher fuel consumption
- Idle quality that changes noticeably in damp weather
Likely causes
- Sensing element coated with oil mist, dust or fuel vapour so it responds slowly or reads flat
- Sensor loose in its mount or positioned so that it does not see the true air stream
- Water standing in the intake duct or a soaked air filter distorting the reading
- High resistance in the signal or ground circuit shifting the voltage enough to skew the value
- Aged element that has drifted outside its calibration
- An incorrect or non matching replacement sensor fitted at an earlier repair
- A fault in one of the reference inputs used for the comparison, such as ambient temperature or barometric pressure
How to fix it
- Compare the reported humidity with the actual weather at that time and place before condemning any part
- Verify that ambient air temperature, barometric pressure and intake air temperature all read correctly, since humidity is judged against them
- Remove and inspect the sensor for oil film, dust or water; a failing crankcase ventilation system is a common source of contamination
- Confirm the sensor sits in its intended position and that no leak allows unfiltered or heated air past it
- Measure the reference voltage, ground and signal at the sensor and check the ground side for voltage drop
- Repair intake leaks or a waterlogged filter housing, then reassess the reading once the intake has dried
- Fit the correct specification sensor only after the supporting data is proven good and the value still fails the plausibility test