What it means
The NH3 sensor circuit signal is below the expected range. This can result from a circuit fault or a sensor fault.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The NH3 sensor measures ammonia in the exhaust so the aftertreatment system can assess SCR emissions control. The control module monitors the sensor signal and sets P24E3 when it falls below the expected range, which can happen if the signal wire is grounded or the connection is compromised.
Common causes
Most likely first.
- Signal shorted to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
Symptoms
- Check-engine light on
- Aftertreatment or emissions warning
- Reduced engine power
- Emissions inspection failure
How to diagnose it
- Record freeze-frame data and check NH3 sensor live data to see when the signal fell low and whether it remains low.
- Inspect the NH3 sensor connector and nearby harness for corrosion, loose pins, or insulation rubbed through to ground.
- Check the signal wire for a short to ground using the wiring diagram and the manufacturer's test procedure.
- Check connector continuity and terminal condition, since corrosion or poor contact can disrupt the signal.
- If wiring and connections pass, test the NH3 sensor and its signal against the manufacturer's specification before replacing it.
Common fixes
- Clean and secure corroded or loose NH3 sensor connector terminals.
- Repair chafed signal-wire insulation and restore the harness routing.
- Repair a signal wire shorted to ground.
- Replace the NH3 sensor if it fails the manufacturer's tests.
Can I keep driving?
The vehicle may remain drivable, but reduced power or an emissions warning can occur and emissions control may be impaired. Avoid demanding driving if power is limited and arrange diagnosis soon; an emissions inspection may fail.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.