What it means
The DEF tank heater control line reads below the expected voltage — short to ground or stuck-low driver.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The reductant tank heater control circuit operates the heater that keeps DEF from freezing in cold conditions. P202B sets when the circuit voltage is below the expected range, which can indicate a short to ground or a control circuit fault.
Common causes
Most likely first.
- Short to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
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
- Check freeze-frame data and reductant heater status, noting any DEF warning or no-start countdown.
- Inspect the heater connector and nearby harness for corrosion, pin damage, or insulation rubbed through to ground.
- Use the wiring diagram to check the control circuit for a short to ground and inspect heater power and ground connections.
- Measure heater resistance and circuit behavior using the manufacturer's diagnostic procedure to distinguish a shorted heater from a wiring fault.
- If the wiring and heater pass, test the control module output according to the manufacturer's procedure.
Common fixes
- Repair chafed control wiring that is shorted to ground.
- Clean or replace corroded heater connector terminals.
- Replace a reductant tank heater that fails resistance or isolation checks.
- Repair the control-side circuit if it remains low with a serviceable heater and harness.
Can I keep driving?
The vehicle may be drivable, but a tank heater that cannot operate can leave DEF frozen and emissions controls impaired in cold weather. Watch for a no-start countdown and arrange repair promptly if one appears.
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.