Code
P248C
Generic
P — Powertrain
Reductant Heater B Sense Circuit Low
AI status
Completed
Completed
100%
Causes
- Open or shorted wiring in Heater B sense circuit (short to ground)
- Corroded, loose, or damaged connector at heater or control module
- Failed/heavily degraded reductant heater B element or internal sense resistor
- Blown fuse or defective relay supplying the heater circuit
- Low battery or poor power/ground affecting sensor readings
- Faulty PCM/ECM or internal sense circuit fault
Symptoms
- MIL (Check Engine lamp) illuminated
- Service message related to DEF/reductant heater or NOx system
- Reduced or disabled reductant heating (poor cold-weather performance)
- Possible engine derate or reduced emissions control performance
- Diagnostic trouble codes related to reductant/heater circuits present
What to check
- Read and record DTC(s), freeze frame and live data for reductant heater B sense line
- Perform a visual inspection of DEF tank, heater assembly, harness and connectors for corrosion, damage or contamination
- Check fuses and relays for the reductant/heater circuit
- With ignition ON (engine off) monitor sense circuit voltage at the PCM and at the heater connector using a DVOM or scope
- Measure heater element resistance at the connector with power removed (compare to spec)
- Perform continuity checks from heater connector to PCM pin and to ground; check for shorts to ground or battery
Signal parameters
- Normal sense voltage (typical): ~0.5–4.5 V (vehicle-specific)
- Trip/Low condition: sense voltage below expected threshold (often
- Heater element resistance (typical): low ohms to tens of ohms — vehicle-specific
- Heater current when active: may be 1–10 A (vehicle-specific)
Diagnostic algorithm
- Obtain freeze frame and note conditions when DTC set. Clear codes and attempt to re-create.
- Visually inspect harness, connectors and DEF heater assembly for damage, corrosion, or fluid intrusion. Repair or clean as required.
- Check related fuses and relays for continuity and proper operation; replace if faulty.
- Disconnect battery or disable circuit power before performing resistance tests. With power removed, measure resistance of Heater B element between its terminals and compare to specification.
- With harness connected and ignition ON (engine OFF), backprobe the heater sense terminal at the PCM and at the heater connector. Verify reference voltage/sense voltage is within expected range.
- If sense voltage is low, test for short to ground on the sense wire (continuity to ground). If short present, isolate by disconnecting sections of harness until short clears.
- If wiring and connectors are good but heater resistance or sense still out of spec, replace the reductant heater assembly (or heater module) and recheck.
- If replacement heater does not correct the condition, check PCM power/grounds and consider upgrading/reprogramming or replacing PCM as a last step.
- After repairs, clear codes and run the vehicle through required readiness/drive cycles and verify heater operation and that the code does not return.
Likely causes
- Short to ground in the sense wire
- Corroded connector pin at heater or module
- Failed heater element or internal sensing component
- Blown fuse or bad relay feeding the heater
- Poor ground or supply to the heater circuit
Fault status
Status
Reductant Heater B Sense Circuit Low — heater B sense voltage below expected range
Repair difficulty: Medium
Diagnostic time: 1.0-2.5 hours
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