What it means
Reductant heater B is electrically OK but does not warm the line in time — partial winding fault or excessive heat loss.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Reductant heater B warms a section of the DEF supply line downstream of heater A. The control module monitors heater B’s electrical operation and warming response, setting this code when the circuit appears functional but the heated line does not respond as expected within the allowed time.
Common causes
Most likely first.
- Heater B partially failed
- Corroded or loose heater connections
- Damaged heater wiring
- Excessive heat loss from the heated line
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
How to diagnose it
- Record freeze-frame temperatures and ambient conditions, then compare the line temperature response with other reductant temperature readings.
- Inspect the heater B line section for damaged insulation, poor routing, loose supports, or other signs of excessive heat loss.
- Check heater B’s connector and wiring for corrosion, looseness, chafing, or heat damage.
- Command heater B on with a scan tool where supported and watch whether the monitored line temperature rises as expected.
- Test heater B’s element and voltage drop under load against the manufacturer’s specification to identify a partial heater or connection fault.
- If the electrical tests pass, investigate heat transfer and temperature sensing for the heater B section using the manufacturer’s procedure.
Common fixes
- Secure or repair loose, corroded, or damaged heater B connections.
- Repair damaged wiring or insulation around the heater B line.
- Replace a weak heater B element or heated line assembly after confirming its performance fault.
- Correct damaged line insulation or routing that causes excessive heat loss.
Can I keep driving?
Driving may be possible in warmer conditions, but heater B may not keep its DEF line section usable during freezing weather. Avoid cold-weather trips until repaired if possible, and heed any no-restart countdown because a blocked or frozen line can prevent SCR dosing.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.