What it means
This code indicates a high voltage condition in reductant heater C's control circuit.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Reductant heater C warms the DEF (AdBlue) in the SCR system so it can flow in cold conditions. The control module monitors heater C’s control circuit and sets P20C4 when it detects voltage higher than expected, which can happen if the circuit is shorted to battery power or has an abnormal connection.
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
- Check the freeze-frame and SCR module data for when P20C4 set, including heater C commands and circuit readings, and note any related heater codes.
- With the system safely cool and powered down as specified by the manufacturer, inspect heater C’s connector and nearby harness for corrosion, pin damage, pinching, or contact with a B+ wire.
- Check the wiring diagram to identify heater C’s control and supply wires, then test for an unintended connection to battery power using the manufacturer’s procedure.
- Inspect connector terminals for poor contact or corrosion that could distort the circuit reading; repair any damaged terminals and retest.
- If the wiring and connections pass, test heater C and its control-module output against the manufacturer’s specification to locate an internal component or driver fault.
Common fixes
- Repair wiring pinched against a B+ source or shorted to battery power.
- Clean or replace corroded or damaged heater C connector terminals.
- Repair damaged harness sections and secure them away from power wires and hot or moving parts.
- Replace heater C or repair the control module only if testing confirms the failed component.
Can I keep driving?
You can usually drive cautiously if the vehicle runs normally, but the DEF system may not work correctly in cold weather and emissions warnings may appear. Do not ignore a no-start countdown; continued operation can lead to emissions inspection failure or a starting restriction.
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.