P00F4
Mass Air Flow Sensor A Circuit LowP00F4 is a generic OBD-II powertrain diagnostic trouble code: Mass Air Flow Sensor A Circuit Low. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P00F4 means
P00F4 is a generic powertrain code set when the Engine Control Module (ECM) detects a low-voltage signal on the intake air humidity sensor circuit. The humidity sensor measures the moisture content of the incoming air, and the ECM uses this input to refine the air-fuel mixture, ignition timing, and on many engines combustion and emissions strategies such as NOx modelling and knock control. When the sensor's returned voltage drops at or below the calibrated low threshold, the ECM interprets it as a circuit fault and stores P00F4 with the MIL illuminated.
A circuit-low condition is most often caused by the signal wire shorting to ground or to sensor ground, an internally shorted sensor, or a corroded connector bridging terminals. Because the humidity sensor is frequently integrated with the intake air temperature sensor, connector contamination and harness damage are common real-world triggers. Aftermarket or low-quality replacement sensors can also produce out-of-range signals.
Drivability effects are usually mild: the ECM substitutes a default humidity value, so most drivers notice only the warning light, perhaps with slightly altered fuelling or a small economy penalty. Diagnosis should start by reading the live humidity value, then confirming reference voltage, ground, and signal-wire integrity before replacing the sensor, since wiring faults mimic sensor failure.
Common causes
Most-frequently reported root causes when P00F4 is logged.
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1
Signal wire shorted to ground or to the sensor ground circuit.
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2
Humidity sensor internally shorted, driving the signal low.
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3
Corroded or contaminated connector bridging signal and ground pins.
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4
Damaged wiring harness in the intake causing an intermittent short.
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5
Faulty integrated intake air temperature and humidity module.
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6
Low-quality aftermarket sensor producing an out-of-range signal.
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7
Water intrusion into the sensor connector.
Symptoms drivers notice
How to diagnose P00F4
A typical diagnostic flow when this code is present.
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1
Read all codes and view the live humidity value with a scan tool.
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2
Inspect the sensor connector for corrosion, contamination, or bridged terminals.
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3
Verify the reference voltage and ground at the sensor connector.
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4
Test the signal wire for a short to ground between the sensor and the ECM.
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5
Compare the humidity reading against ambient conditions for plausibility.
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6
Substitute a known-good sensor only after wiring checks pass, then clear and retest.
Related powertrain codes
Frequently asked questions
What does P00F4 refer to?
It is a low-voltage fault on the intake air humidity sensor circuit, not the mass air flow sensor.
What usually causes a circuit-low reading?
A short to ground in the signal wiring, an internally shorted sensor, or a corroded connector bridging terminals are the most common causes.
Is P00F4 serious?
It is generally low severity because the ECM uses a default humidity value, but it should be repaired to restore accurate fuelling and emissions control.
Can a cheap replacement sensor set P00F4?
Yes, low-quality aftermarket humidity or combined temperature/humidity sensors can output signals outside the expected range and trigger the code.
Disabling P00F4 in software
RaceTune can permanently disable P00F4 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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