What does it mean?
Modern engine management uses a humidity sensor to measure how much water vapour the incoming air carries, because moist air holds less oxygen per unit volume and burns more slowly. Gasoline engines use the value to trim spark advance and knock protection, while diesels use it to set exhaust gas recirculation rates and NOx targets. This code means the module saw no usable circuit at all: the signal or reference path reads as an open, so the sensor cannot report anything. A complete loss of continuity is an electrical fault rather than a drifted measurement, and the module substitutes a default humidity figure until the circuit is restored.
Symptoms
- MIL (check engine lamp) on, with the humidity value missing or frozen at a substitute figure on a scan tool
- Slightly reduced power because the module applies a conservative spark or boost strategy
- On diesels, more visible smoke or higher exhaust temperature as the EGR target falls back to a default
- Cold or damp weather starts that are rougher than usual
- A small rise in fuel consumption
- Adaptive functions such as knock control staying in a protective mode
Likely causes
- Broken signal or reference wire between the sensor and the control module
- Connector left disconnected or only partly seated after intake or air filter housing service
- Corroded terminals from water entering the connector where the sensor sits in the inlet duct
- Open sensor ground circuit or a corroded ground splice
- Internally failed sensing element, sometimes after contamination by crankcase oil mist
- Broken connector lock allowing the terminals to separate under vibration
- Rarely, a failed reference voltage output inside the control module
How to fix it
- Confirm the fault with a scan tool and check whether the humidity reading is absent or locked to a fixed default
- Inspect the sensor connector for backed out terminals, moisture and green corrosion, then reseat it fully
- Measure the reference voltage at the sensor connector with the key on; no reference points upstream, a good reference points downstream
- Test continuity and resistance of the signal and ground wires back to the module connector while flexing the harness
- Repair broken conductors and corroded splices properly rather than taping over them, and seal the connector against water
- Replace the sensor when supply, ground and signal wiring all measure good but the circuit still reads open
- Clear the code and drive the vehicle through conditions similar to the freeze frame to verify the repair