What it means
The reductant tank heater control circuit voltage is higher than expected. The code identifies a high circuit condition; the specific wiring or component fault depends on the vehicle.
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 switches the heater that keeps DEF usable in freezing conditions. P202C sets when the circuit voltage is higher than expected, which can result from a short to battery power, a connection fault, or a control-side problem.
Common causes
Most likely first.
- Short to B+
- Connector pins corroded - high contact resistance
- Supply wire pinched against B+ source
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
- Review freeze-frame data and reductant heater status, and note any DEF, NOx, or start-countdown warnings.
- Inspect the tank-heater connector for corrosion, loose pins, or damage that could bridge the circuit to a power feed.
- Trace the heater control wiring for pinching or insulation damage where it could contact battery voltage.
- Check for a short to power and verify heater and circuit behavior using the manufacturer's wiring diagram and specifications.
- If wiring and heater checks pass, test the control module output according to the manufacturer's procedure.
Common fixes
- Repair wiring that is shorted to a power source.
- Clean or replace damaged heater connector terminals.
- Repair a pinched or damaged section of the heater control harness.
- Replace a faulty tank heater or repair the control-side circuit if testing isolates the fault there.
Can I keep driving?
The vehicle may remain drivable, but the heater may not control correctly, leaving DEF vulnerable to freezing and emissions functions impaired. Monitor warnings and any no-start countdown, and address the fault promptly.
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.