What it means
The NH3 sensor heater circuit signal is above the expected range. The code does not by itself identify the cause or the control module’s response.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The NH3 sensor heater brings the sensing element to operating temperature for exhaust ammonia measurement. P24E8 means the control module detects heater-circuit voltage above its expected range, which may occur if the control wire contacts battery voltage or the driver remains switched on.
Common causes
Most likely first.
- Heater control wire shorted to battery voltage
- High-side driver stuck on
- Connector pins corroded - high contact resistance
Symptoms
- Check-engine light on
- Aftertreatment or emissions warning
- Reduced engine power
- Emissions inspection failure
How to diagnose it
- Save freeze-frame data and review heater-circuit status and related codes to establish when the high condition is detected.
- Inspect the NH3 heater connector and harness for corrosion, pin damage, or wiring touching a battery-voltage source.
- Follow the wiring diagram to test the heater control wire for a short to battery voltage.
- Check connector terminal condition and heater-circuit continuity using the manufacturer's procedure.
- If the circuit wiring and sensor pass, test whether the control module driver is stuck on.
Common fixes
- Clean and secure corroded or loose heater-circuit terminals.
- Repair wiring that is pinched against a battery-voltage source.
- Repair a heater control wire shorted to battery voltage.
- Replace the NH3 sensor or repair the driver only if testing confirms that component has failed.
Can I keep driving?
The vehicle may still run, but the heater may be controlled incorrectly and emissions monitoring can suffer; reduced power or an aftertreatment warning may follow. Have it checked soon, and stop driving if an electrical burning smell or visible harness damage is present.
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.