P24E8: NH3 Sensor Heater Circuit High

With P24E8 the heater circuit of the NH3 sensor reads too high. Either the line is shorted to a supply voltage so that the heater cannot be switched off, or the driver measures overcurrent because the element has failed with low resistance. A heater stuck on can overheat and crack the ceramic, so the controller normally shuts it down and stops evaluating ammonia until the fault clears. No manufacturer documents P24E8 publicly. CARB's heavy-duty regulation treats heater current or voltage drop outside the specified limits as a malfunction. Navistar's N13 shows that faults of this heater are reported by the smart sensor itself. P24E7 is the low counterpart, P24E6 the open circuit.

Medium severity. Can I keep driving?If the car is running normally, drive in moderation to a repair shop. If it loses power or another warning appears, do not continue.

Symptoms

Malfunction indicator lamp on
Ammonia sensor reading switched off

Most likely causes

Heater control line shorted to battery voltage
Heater element failed with low resistance, causing overcurrent
Heater driver in the sensor module defective

What to check, in order

Start with the simplest. You can tick off each step.

Check the battery, connectors, and wiring
Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
Read the live data with a scanner
Follow the manufacturer's service manual

Good to know

Repair difficulty: medium
It affects emissions.

What each character of P24E8 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
4: Emissions
E8: Specific fault in that system

Frequently asked questions

Can I keep driving with code P24E8?

If the car is running normally, drive in moderation to a repair shop. If it loses power or another warning appears, do not continue.

What causes code P24E8?

The most common causes are: heater control line shorted to battery voltage; heater element failed with low resistance, causing overcurrent; heater driver in the sensor module defective.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.