P24E2 flags an NH3 sensor whose signal is electrically valid but makes no sense: the ammonia value sticks, drifts or contradicts what the dosing model expects from the SCR catalyst. On heavy trucks the sensor is a smart unit with its own electronics that test probe and circuitry and report over J1939. Navistar lets this self-test run on the 2013 N13 only after the middle of the SCR brick has stayed above 170 °C for five seconds, and lights the MIL after two failed trips. No manufacturer publishes P24E2 with thresholds. CARB's heavy-duty rule obliges makers to check such sensors for rationality. Wiring faults are P24E3/P24E4, dropouts P24E5. The driver sees a warning lamp; DEF dosing becomes less precise.
Medium severity. Can I keep driving?If the car is running normally, drive in moderation to a repair shop. If it loses power or another warning appears, do not continue.
Symptoms
- Check engine light after two trips
- DEF consumption noticeably higher or lower than usual
- Possible ammonia smell at the tailpipe
Most likely causes
- Sensing element aged or poisoned by deposits, giving a drifting ammonia value
- Urea crystals around the sensor tip after overdosing
- Exhaust leak near the sensor diluting the sample
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P24E2 means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 4: Emissions
- E2: Specific fault in that system
Frequently asked questions
Can I keep driving with code P24E2?
If the car is running normally, drive in moderation to a repair shop. If it loses power or another warning appears, do not continue.
What causes code P24E2?
The most common causes are: sensing element aged or poisoned by deposits, giving a drifting ammonia value; urea crystals around the sensor tip after overdosing; exhaust leak near the sensor diluting the sample.
Related codes
- P24E1NH3 Sensor Circuit
- P24E3NH3 Sensor Circuit Low
- P24E4NH3 Sensor Circuit High
- P24E5NH3 Sensor Circuit Intermittent/Erratic
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.