Powertrain, generic (extended)

P24E4 NH3 Sensor Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

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.

Likely causes

Signal line shorted to a supply voltage in the harness
Open line between probe and module read as maximum value
Sensor electronics module outputting a saturated value

Symptoms

  • Check engine light on
  • Ammonia reading pinned at the upper limit
  • Possible reduced DEF dosing

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€100–1,400
Time0.5–2.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

nh3 sensor

Related codes

P24E1P24E2P24E3P24EC

References: law.cornell.edu, static.nhtsa.gov.