P248D: Reductant Heater B Sense Circuit High

P248D means the heater B sense input reads too high, typically the heated line. Possible pictures are a sense wire shorted to battery positive, a supply that remains on after the heater has been switched off, or a faulty input stage. Public maker documentation for P248D does not exist. GM tests the line heater sense circuit only for undervoltage (P248C); Ford's 2023 diesels cover heater circuit 2, the pressure line heater, with P20C0 (switched off yet more than 5 V at the output) and the current codes P221C/P221D. CARB requires the reductant heating to be monitored. A line heater that never switches off wastes battery energy and can overheat the hose, so the relay is the first item to check.

Symptoms

• Warning lamp
• Battery discharging while parked

Most likely causes

• Line heater relay sticking closed
• Sense wire shorted to battery positive
• Defective input stage in the module

What to check, in order

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

1. Check the AdBlue/DEF level and quality, and do not add additives or water
2. Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
3. Look for white crystals around the injector and the tank
4. Check connectors and heaters with the scanner
5. Run the dosing test available on your diagnostic tool
6. Replace parts only when the test points to that component

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P248D means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 4: Emissions
• 8D: Specific fault in that system

Frequently asked questions

Can I keep driving with code P248D?

Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

What causes code P248D?

The most common causes are: line heater relay sticking closed; sense wire shorted to battery positive; defective input stage in the module.

Related codes

• P248AReductant Heater A Sense Circuit Low
• P248BReductant Heater A Sense Circuit High
• P248CReductant Heater B Sense Circuit Low
• P20C0Reductant Heater B Control Circuit High
• P221CReductant Heater B Current Too Low

Sources

• law.cornell.edu: 13 CCR 1968.2
• gsi.ext.gm.com: 18OBDG04 Engine Diagnostics
• fordservicecontent.com: obd summary diesel 2023

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.