P24E4 code meaning
NH3 Sensor Circuit High
Powertrain system Generic code P24xx codes Check engine light
What P24E4 means
P24E4 is set when the NH3 sensor signal climbs above anything a working sensor could report, so the value is rejected as electrically impossible rather than merely implausible. Typical reasons are a short to a supply voltage, a broken line pulled up internally, or failing electronics that output a saturated value. In Navistar's layout the ammonia measurement comes from a probe at mid-bed of the SCR catalyst plus a separate control module, so a reading at the upper stop usually points to the link between the two or to that module. No manufacturer publishes thresholds for P24E4. CARB's heavy-duty OBD rule calls for range checks on such sensors. P24E3 is the low fault; the general circuit code is P24E1.
P24E4 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 2.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P24E4
- Check engine light on
- Ammonia reading pinned at the upper limit
- Possible reduced DEF dosing
Common causes of P24E4
- Most likelySignal line shorted to a supply voltage in the harness
- PossibleOpen line between probe and module read as maximum value
- Less likelySensor electronics module outputting a saturated value
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Nh3 sensor
What each part of P24E4 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 4 | Area: auxiliary emission controls (catalytic converter, EGR, EVAP). |
| E4 | The exact fault in that area. |
What to do next
- Note P24E4 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P24E4
What does P24E4 mean?
P24E4 means "NH3 Sensor Circuit High". P24E4 is set when the NH3 sensor signal climbs above anything a working sensor could report, so the value is rejected as electrically impossible rather than merely implausible. It is a generic code, so it means the same on every make.
Is it safe to drive with P24E4?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P24E4 fail an emissions test?
Yes, it can. P24E4 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P24E4 to fix?
Difficulty: medium. A typical repair takes 0.5 to 2.5 hours of work. This is a job for a workshop.
Will P24E4 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.