P24E3

Code

Generic P — Powertrain

NH3 Sensor Circuit Low

Brand: Generic

Type: P — Powertrain

Views: UK: 19 EN: 32 RU: 21

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Page language: EN

Causes

Failed or degraded NH3 sensor
Open or shorted wiring in the sensor signal, reference or ground circuit
Corroded, loose or damaged sensor connector
Failed sensor heater or blown heater fuse (when heater affects sensor output)
Poor ground or low reference voltage from the ECM
ECM internal fault or software issue

Symptoms

Malfunction Indicator Lamp (MIL) / check engine light ON
Reduced SCR/aftertreatment control or active limp mode for emissions
Failed emissions test (elevated NOx/HC/AMMONIA slip)
Possible regen or aftertreatment system warnings/messages
Occasional or persistent diagnostic trouble codes related to SCR/NH3 system

What to check

Scan for stored/freeze-frame data and any related DTCs (NOx, SCR, heater codes)
Visually inspect sensor, connector, and harness for damage or corrosion
Backprobe sensor connector to measure signal, reference and ground voltages with key ON
Verify heater supply voltage and check heater resistance per service data
Check continuity between sensor ground and chassis/ECM ground
Wiggle harness while monitoring live data to reproduce fault

Signal parameters

Typical NH3 sensor output: 0–5.0 V (varies by design); a low condition is generally a voltage near 0 V — consult manufacturer specs
Sensor heater supply: ~12 V with key ON/engine running (depends on design); heater resistance typically low (consult manual)
Reference circuit: may use a 5 V reference or ECM internal bias — should be stable and within spec
Continuity: signal and ground circuit should show low resistance to ECM pins; open/very high resistance indicates a break
Note: exact values and thresholds are vehicle/manufacturer specific — always verify against service data

Diagnostic algorithm

Retrieve DTCs and freeze-frame data; note related codes for SCR, NOx, or heater circuits.
Inspect sensor and harness for physical damage, corrosion, crushed wires, or exhaust leaks near sensor.
With connector connected, monitor live data (sensor voltage, heater status) using a scan tool. Record idle and under-load values if available.
With key ON (engine OFF), backprobe the signal, reference and ground pins at the sensor connector. Verify reference voltage and ground continuity; check for a measurable signal voltage.
Measure heater supply voltage with key ON/engine running and measure heater resistance with sensor disconnected. Compare to manufacturer spec.
Perform continuity/resistance checks between the sensor pins and the ECM connector to look for opens or high resistance. Repair any wiring faults.
If wiring and supplies are within spec but signal remains low, replace the NH3 sensor and reset codes.
After repair or replacement, clear codes, perform a road test or required relearn/regeneration procedures, and re-scan to confirm the fault is cleared.
If fault returns after replacement, consider ECM diagnostics/repair or deeper bench testing of the sensor/ECM harness.

Likely causes

Broken/shorted wire between NH3 sensor and ECM (signal or ground)
Corroded connector terminals at sensor
Sensor internal failure (element or electronics)
Loss of sensor heater supply or blown fuse causing abnormal sensor output
Poor chassis/ECM ground affecting sensor reference

Fault status

⚠️ Status

NH3 Sensor Circuit Low — the ammonia sensor signal is lower than expected. Possible causes include a failed sensor, open/short in signal/ground, poor reference voltage, or heater/supply fault. Verify wiring, connector, power/ground, and sensor function.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.5 hours