P24E4: NH3 Sensor Circuit High

What it means

The NH3 sensor circuit signal is above 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 reports ammonia levels in the exhaust to help the aftertreatment system monitor SCR operation. The control module monitors its signal and sets P24E4 when that signal rises above the expected range, which can result from a short to supply voltage or a faulty connection.

Common causes

Most likely first.

1. Signal shorted to supply
2. Connector pins corroded - high contact resistance
3. Supply wire pinched against B+ source

Symptoms

• Check-engine light on
• Aftertreatment or emissions warning
• Reduced engine power
• Emissions inspection failure

How to diagnose it

1. Record freeze-frame data and check NH3 sensor live data to see whether the signal stays high or rises intermittently.
2. Inspect the NH3 sensor connector and harness for corrosion, pin damage, or a wire pinched against a power source.
3. Use the wiring diagram and manufacturer's procedure to check whether the signal wire is shorted to supply voltage.
4. Check connector terminal fit and continuity, since corrosion can interfere with the sensor signal.
5. If the harness and connector pass, test the NH3 sensor against the manufacturer's specification.

Common fixes

• Clean and secure corroded or loose connector terminals.
• Repair a pinched or chafed wire contacting a supply source.
• Repair a signal wire shorted to supply voltage.
• Replace the NH3 sensor if it fails the manufacturer's tests.

Can I keep driving?

The vehicle may be drivable, but reduced power or an aftertreatment warning may develop and emissions control may be affected. Avoid strenuous driving if power is limited, and have the fault checked soon because the vehicle may fail an emissions inspection.