P20C4 code meaning
Reductant Heater C Control Circuit High
Powertrain system Generic code P20xx codes Check engine light
What P20C4 means
P20C4 marks heater C's control circuit as high, meaning it sees battery voltage where none belongs. GM's 2025 tables test heater 3, a second tank heater, with the output switched off against a raised voltage window and set a Type A fault after 2 s of persistence. GM's LML documentation from 2014 (6.6 L) has the glow plug module react to over 3.5 V on the heater pin while the heater should be off (Type B). Deutz lists its return line heater with FMI 3, voltage above normal. No public service bulletin names P20C4, so the main suspects are harness damage near a supply wire and an output stage that stays on. An element that keeps heating with the engine off drains the battery; once the module locks it out, that section freezes. Open and low faults of heater C are P20C1 and P20C3.
P20C4 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P20C4
- Check engine light and SCR warning
- Battery drained overnight if the heater stays powered
- Inducement and speed limit if left unrepaired
Common causes of P20C4
- Most likelyHeater C control wire in contact with a battery feed
- PossibleHeater driver in the DEF or glow plug module stuck on
- Less likelyDEF crystals or moisture bridging pins in the heater connector
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Reductant line heater c
What each part of P20C4 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 0 | Area: fuel and air metering and auxiliary emission controls. |
| C4 | The exact fault in that area. |
What to do next
- Note P20C4 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P20C4
What does P20C4 mean?
P20C4 means "Reductant Heater C Control Circuit High". P20C4 marks heater C's control circuit as high, meaning it sees battery voltage where none belongs. It is a generic code, so it means the same on every make.
Is it safe to drive with P20C4?
Drive gently and only to a workshop. With P20C4 the car can go into limp mode (reduced power) to protect itself.
Will P20C4 fail an emissions test?
Yes, it can. P20C4 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P20C4 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will P20C4 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.