P24E1

Code

Generic P — Powertrain

NH3 Sensor Circuit

Brand: Generic

Type: P — Powertrain

Views: UK: 20 EN: 41 RU: 21

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Page language: EN

Causes

Open or shorted sensor signal wire
Corroded, loose, or damaged connector
Failed NH3 sensor element
Heater circuit open/short (if sensor has heater)
Low supply voltage or poor ground to sensor
Contamination or fouling of sensor (urea crystallization)

Symptoms

Malfunction Indicator Lamp (MIL) or dealer warning lamp illuminated
Reduced or limited SCR system performance; possible reduced engine power or limp mode
Failed emissions/NOx control readiness or failed emissions test
Increased NOx tailpipe emissions
SCR dosing abnormalities or frequent DEF system messages
Possible active DPF/regeneration inhibit if system interlocks

What to check

Read freeze frame and live data with a capable scan tool (NH3 ppm, sensor status, heater status, supply voltages)
Visual inspection of sensor, connector, and wiring for damage, corrosion, or contamination
Check for DEF/urea crystallization around sensor and SCR components
Measure sensor supply voltage and ground at connector with key on, engine off
Measure heater resistance (if applicable) and compare to manufacturer spec
Backprobe signal wire while monitoring live NH3 value and compare to expected behavior during dosing

Signal parameters

Sensor signal: typically low-voltage analog output proportional to NH3 concentration (manufacturer-specific — often 0–5 V or 0–1 V range)
Heater circuit: resistance typically low ohms (check manufacturer spec); draws switched 12 V when heating
Expected idle NH3 levels: near zero ppm with no dosing; rises transiently during active dosing (use manufacturer live-data ranges)
Response time: sensor should change output quickly when exposed to known NH3 source; slow/no response indicates sensor or heater issue
Supply: switched +12 V and clean ground required; voltage should be within battery system range

Diagnostic algorithm

Confirm DTC and note freeze frame and related codes. Clear code and perform a drive/operation cycle to see if it returns.
Visually inspect sensor, connector, and wiring for damage, corrosion, cuts, or signs of DEF crystallization. Repair as needed.
With ignition on, measure supply voltage and ground at sensor connector. Verify proper battery voltage and a good ground reference.
Check heater circuit (if present): measure resistance at sensor and verify heater supply activation via scan tool or by energizing circuit. Replace sensor if heater open or shorted.
Backprobe the signal wire and monitor live NH3 sensor output while commanding DEF dosing or exposing sensor to a controlled NH3 source per service info. Confirm sensor responds within expected range and time.
Wiggle-test wiring and connectors while monitoring signal to find intermittent faults. Repair harness and connectors as required.
If wiring and heater are good but signal is out of range or non-responsive, replace the NH3 sensor with a known-good unit and recheck.
If fault persists after sensor replacement, perform ECU connector and pin voltage checks. Consider ECU repair or replacement only after verifying all external circuits are good.
After repairs, clear codes and complete required relearn/calibration procedures and a verification drive cycle to confirm repair.

Likely causes

Broken or corroded connector at the sensor
Sensor element failure from contamination or age
Heater circuit failure preventing sensor from reaching operating temperature
Short to battery or ground in the signal wire
Loose or high resistance ground or supply feed
ECU pin fault or internal module failure (less common)

Fault status

⚠️ Status

NH3 sensor circuit fault detected — possible open/short, heater failure, contaminated sensor, poor supply/ground, or intermittent wiring. Inspect sensor, wiring, heater circuit, and related modules.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours