P24E2 NH3 Sensor Circuit Range/Performance

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.

Likely causes

Symptoms

• Check engine light after two trips
• DEF consumption noticeably higher or lower than usual
• Possible ammonia smell at the tailpipe

Repair estimate

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

Parts involved

Related codes

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