Powertrain, generic (extended)

P24EB NH3 Sensor Supply Voltage Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

P24EB reports that the power feed of the NH3 sensor module sags below its lower limit. Below that point the heater cannot bring the probe up to temperature and the electronics may reset, so ammonia readings and sensor messages become unreliable. Typical causes are a weak or discharged battery during cranking or long idle, a high-resistance ground strap, corroded fuse contacts or voltage drop along a long chassis harness. No manufacturer documents P24EB publicly; Navistar's N13 material shows the ammonia sensor as a separately powered probe-and-module unit. For heavy-duty engines, CARB wants such circuits checked for undervoltage. P24EA is the general supply fault, P24EC the high one.

Likely causes

Weak or discharged batteries pulling system voltage down
High resistance in the ground path of the sensor module
Corroded fuse or relay contacts causing voltage drop

Symptoms

  • Warning lamp, often after cold starts
  • NH3 sensor drops off the data link at times

Repair estimate

DifficultyEasyDIY possible
Parts and labour€30–600
Time0.3–2 h

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

Parts involved

nh3 sensorsupply wiring

Related codes

P24EAP24ECP24E3

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