P24E7: NH3 Sensor Heater Circuit Low

P24E7 points to a heater circuit of the NH3 sensor that reads too low: the feedback voltage stays near ground, or the current is far below what the heater should draw. Most often the heater feed is shorted to ground, which can also blow the fuse or switch off the driver to protect it; a heater whose resistance has risen sharply gives a similar picture. The probe then heats slowly or not at all, and the ammonia value stays unavailable. No manufacturer publishes P24E7 with thresholds. CARB's heavy-duty OBD rule demands that heater current or voltage drop outside the maker's limits be flagged. The open-circuit fault is P24E6, the high fault P24E8 and heating performance P24E9.

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

Check engine light on
Ammonia value missing or delayed after start

Most likely causes

Heater supply line shorted to ground
Heater element with strongly increased resistance
Blown fuse in the heater supply

What to check, in order

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

Check the battery, connectors, and wiring
With the connector unplugged, check whether the signal wire is making contact with ground.
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 P24E7 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
4: Emissions
E7: Specific fault in that system

Frequently asked questions

Can I keep driving with code P24E7?

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 P24E7?

The most common causes are: heater supply line shorted to ground; heater element with strongly increased resistance; blown fuse in the heater supply.

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.