P24E8

Code

Generic P — Powertrain

NH3 Sensor Heater Circuit High

Brand: Generic

Type: P — Powertrain

Views: UK: 20 EN: 39 RU: 20

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Short to battery/constant 12V on the heater circuit
Faulty NH3 sensor (internal heater failure)
Wiring short or damaged harness (insulation chafing, pinched, exposed conductor)
Corroded/poor connector or terminal making incorrect contact
Failed heater control relay or driver in PCM/ECM
Blown or incorrect fuse, aftermarket wiring fault

Read and record freeze-frame and any related DTCs with a scan tool
Visual inspection of NH3 sensor connector, wiring harness and mounting for damage, corrosion, or water intrusion
Check fuse(s) and any heater relay for proper operation
Measure battery voltage and verify proper supply to the heater circuit
With ignition off, measure heater resistance across the sensor heater pins and compare to spec
Backprobe heater supply and ground while commanding heater ON to measure voltage and current

Heater supply voltage: ~battery voltage when heater commanded ON (typically 11–14 V)
Heater resistance: varies by sensor design; low-ohm element (often 1–50 Ω). Refer to OEM spec
Heater current draw: typically up to a few amps depending on resistance (measure with clamp meter)
ECM commanded duty cycle or heater ON/OFF status (scan tool live data)
Timeout/threshold parameters vary by manufacturer; fault trips when measured values exceed expected range

Retrieve DTCs and freeze-frame data. Note whether fault is current or historic and any related codes (NOx system, sensors, fuses).
Perform a careful visual inspection of the NH3 sensor, connector, and wiring for damage, corrosion, broken pins, or water ingress. Repair any obvious faults.
Check related fuses and relays for the heater circuit; replace if open or suspect.
With ignition OFF, disconnect the sensor and measure the resistance across the heater pins. Compare to OEM specification. An open or very low (near short) reading indicates sensor failure.
With harness connected, backprobe the heater supply and ground. Command the heater ON using a scan tool (if supported) and measure voltage and current. If supply is higher than expected or always present, look for short to battery.
Inspect and test continuity of wiring between the sensor connector and the ECM/relay. Check for short to battery (voltage on harness with ignition off) or short to ground.
If wiring and connectors test good but sensor heater readings are out of spec, replace the NH3 sensor and clear codes. Re-test to verify fault does not return.
If fault persists after sensor replacement and wiring checks, test/replace the heater driver/relay or consult ECM diagnostics for possible module fault.
After repairs, clear codes and perform road/operation cycle to confirm proper operation and that the DTC does not reappear.