What it means
Reductant heater B (next supply-line segment) control circuit open — that line segment will not be kept above freezing.
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 downstream section of the DEF supply line so fluid can reach the SCR dosing system in cold weather. The control module monitors heater B’s electrical path and sets this code when it detects an open circuit, meaning current cannot flow through the expected path.
Common causes
Most likely first.
- Heater winding open
- Pin pushed back out of connector body
- Cracked solder joint inside the module or sensor
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 data and check for other reductant heater codes to see whether the fault is limited to heater B.
- Locate the heater B section using the vehicle’s wiring information, then inspect its connector for a backed-out pin, corrosion, or a loose lock.
- Check the heater B harness along the supply-line segment for breaks, chafing, or damage near clips and bends.
- With the circuit safely isolated as specified by the manufacturer, test heater B’s winding for continuity and compare with the manufacturer’s specification.
- If the wiring and heater winding pass, check the module or sensor assembly connections for an internal open or cracked joint using the prescribed procedure.
Common fixes
- Reseat a pushed-back heater B connector pin and secure the connector.
- Repair an open or damaged wire in the heater B harness.
- Replace the heater B element or heated line assembly if its winding is open.
- Repair or replace a module or sensor assembly only if the internal open is confirmed.
Can I keep driving?
The vehicle may be drivable in mild weather, but the supply-line section served by heater B may not stay warm enough for DEF flow in freezing conditions. Follow any NOx warning or no-restart countdown; ignoring it can lead to a restart restriction and the SCR system may not control emissions properly.
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.