P24E4

Code

Generic P — Powertrain

NH3 Sensor Circuit High

Brand: Generic

Type: P — Powertrain

Views: UK: 17 EN: 33 RU: 22

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Page language: EN

Causes

Open or shorted sensor signal wire (short to battery positive)
Poor or corroded connector or pin damage at the sensor or ECM
Failed NH3 sensor (internal electronics or heater fault)
Power supply (B+) stuck high or shorted to sensor signal
ECM input circuit fault or software anomaly
Contamination or thermal damage of the sensor from hot exhaust or chemical attack

Symptoms

Malfunction indicator lamp (MIL) or SCR warning illuminated
Reduced aftertreatment performance or derate of SCR dosing
Regeneration or urea dosing faults / limited dosing commands
Ammonia (rotten egg) smell from exhaust in some cases
Vehicle may store related aftertreatment fault codes

What to check

Read freeze frame and stored data with a capable scan tool; record NH3 sensor voltage, SCR dosing status, and related temps
Visually inspect sensor, connector, and wiring for damage, corrosion, or loose pins
Check for aftermarket or recent repairs near the SCR/aftertreatment that could have disturbed wiring
Verify battery voltage and look for parasitic or wiring shorts to battery positive
Check for stored heater circuit codes and verify heater supply and ground

Signal parameters

Typical NH3 sensor signal: 0–5.0 V (manufacturer-dependent)
Normal idle/exhaust low-ammonia: low voltage near 0–1 V; high ammonia concentration increases voltage (specs vary)
Heater supply: full battery voltage when commanded (check resistance per service specs)
If signal reads near battery voltage or stuck high, it indicates a high/short-to-B+ condition

Diagnostic algorithm

Capture live data and related codes; confirm P24E4 is current and note freeze frame values.
Perform a thorough visual inspection of the NH3 sensor, connector, and harness back to the SCR module for chafing, heat damage, or corrosion. Repair any damage found.
With ignition on (engine off) back-probe the sensor signal and compare to expected idle/reference. If signal is near battery voltage, suspect short to B+ or internal sensor short.
Check sensor ground and B+ supply circuits for proper continuity and no shorts: disconnect connector and measure between signal pin and B+ / ground to identify unintended shorts.
Test heater circuit resistance per manufacturer spec; verify heater power only when commanded by ECM. Replace sensor if heater open or abnormal.
Wiggle wiring and connectors while watching live data for intermittent changes to reproduce fault.
If wiring and connectors test OK, substitute a known-good NH3 sensor (if available) or replace the sensor and recheck.
If problem persists after sensor and wiring replacement, consult OEM wiring diagrams and test/replace ECM or associated modules as directed by lab/manual diagnostics.
Safety note: SCR components run very hot — allow the vehicle to cool and wear appropriate PPE when accessing sensors or exhaust components.

Likely causes

Damaged or pinched wiring harness near exhaust/SCR module
Corroded connector at NH3 sensor (water/road salt ingress)
NH3 sensor internal failure (common afterservice life)
Connector swapped or poor repair after prior service around aftertreatment

Fault status

⚠️ Status

ECM has detected an abnormally high voltage or signal from the NH3 sensor circuit (indicating possible short-to-B+, sensor or harness failure) and set P24E4.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours

Repair manuals