P248C: Reductant Heater B Sense Circuit Low

P248C reports too little voltage on the sense circuit of DEF heater B. In GM's diesel tables from 2016 to 2018 heater B is the line heater, i.e. the heated hose from the pump to the dosing valve. The test mirrors P248A: if system voltage exceeds the measured line heater supply by more than 3 V while heating is requested, 10 failures in 12 checks at 500 ms set the code, with cranking excluded and U010E as a blocking condition; no lamp type is listed. Ford's 2023 diesels likewise give the pressure line heater the letter B. A frozen hose that does not thaw causes dosing faults in winter, so a voltage drop at the line heater plug or in its fuse and relay path should be found early.

Symptoms

• DEF warning or engine lamp
• Dosing faults at low temperatures

Most likely causes

• Corroded plug at the heated DEF line
• Voltage drop across fuse or relay of the line heater
• Damaged supply lead along the frame

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. With the connector unplugged, check whether the signal wire is making contact with ground.
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 P248C means

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

Frequently asked questions

Can I keep driving with code P248C?

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

What causes code P248C?

The most common causes are: corroded plug at the heated DEF line; voltage drop across fuse or relay of the line heater; damaged supply lead along the frame.

Related codes

• P248AReductant Heater A Sense Circuit Low
• P248BReductant Heater A Sense Circuit High
• P248DReductant Heater B Sense Circuit High
• P20BDReductant Heater B Control Circuit/Open
• P20BEReductant Heater B Control Circuit Performance

Sources

• gsi.ext.gm.com: 18OBDG04 Engine Diagnostics
• gsi.ext.gm.com: 17OBDG04 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.