P248A: Reductant Heater A Sense Circuit Low

P248A concerns the sense line with which the control unit measures the supply voltage reaching DEF heater A, and reports that voltage as too low. GM uses it on its diesels with an SCR power module for the tank heater: the 2016 to 2018 tables subtract the heater supply voltage from system voltage and fail a gap of more than 3 V while heating is commanded, after 10 of 12 checks at 500 ms; cranking is excluded and a lost connection to the reductant module (U010E) blocks the test. The tables leave the lamp column blank. Ford's 2023 diesels also assign heater A to the tank. A voltage drop of this size usually comes from a corroded plug, a weak relay contact or a damaged supply lead.

Symptoms

• Warning lamp or DEF system message
• Frozen DEF not thawed in winter, dosing delayed

Most likely causes

• Corroded heater supply connector at the tank
• Worn heater relay contact causing voltage drop
• Weak battery or charging fault during heating

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 P248A means

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

Frequently asked questions

Can I keep driving with code P248A?

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

What causes code P248A?

The most common causes are: corroded heater supply connector at the tank; worn heater relay contact causing voltage drop; weak battery or charging fault during heating.

Related codes

• P248BReductant Heater A Sense Circuit High
• P248CReductant Heater B Sense Circuit Low
• P248DReductant Heater B Sense Circuit High
• P20B9Reductant Heater A Control Circuit/Open
• P20BBReductant Heater A Control Circuit Low

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.