What it means
Engine Coolant Heater B Control Circuit Low: the control-circuit voltage is below the expected range. Often a short to ground or high resistance in the wiring.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Engine Coolant Heater B warms coolant through a circuit controlled or monitored by the engine module. This code means the module sees the control-circuit signal for heater B below its expected range, which can happen if the circuit is grounded or has excessive resistance.
Common causes
Most likely first.
- Wiring shorted to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
Symptoms
- Check-engine light or coolant-heater warning light
- Longer engine warm-up in cold weather
- Reduced cabin heat during warm-up
How to diagnose it
- Check freeze-frame and live data for heater B to confirm when the low signal was detected and whether the heater was commanded on.
- Inspect heater B's wiring where it passes near hot, moving, or sharp parts for chafing or insulation worn through to ground.
- Disconnect and examine the heater B connector for corrosion, moisture, or terminals that fit loosely.
- Test the control wire for an unintended path to ground and check its connections for excessive resistance, following the manufacturer's procedure.
- If the harness and connector pass inspection, check the heater element and control-module output for a fault.
Common fixes
- Clean and secure corroded or loose heater B connector terminals.
- Repair chafed wiring that is shorting heater B's control circuit to ground.
- Replace damaged connector terminals or the affected harness section.
- Replace a confirmed faulty heater element or repair a failed control-module driver.
Can I keep driving?
You can usually drive the vehicle, but heater B may not warm the coolant, causing slower warm-up and weaker cabin heat in cold weather. Allow extra time for the engine and windows to warm and clear.
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.