P20C1: The engine computer found an open circuit in the third (C) heater control circuit for the diesel exhaust fluid (DEF) system
Reductant heater C control circuit open
Severity: Inspect soon
The check engine and DEF lights come on, and GM diesel owners report the truck being speed-limited by the emissions system, so get it checked soon.
It gets more serious if:
- Stop: Pull over somewhere safe if the truck starts a speed-limit countdown or drops into limp mode; owners report speed cut to 5 mph, and in extreme cold the DEF system may not thaw.
P20C1 in short
P20C1 means the computer found an open circuit in the C reductant (DEF) heater control. Owners of 2020-2021 GM diesel pickups report a DEF light, check engine light and a speed limit, with one dealer finding a melted DEF tank connector. Have it diagnosed promptly, since the speed limit can follow.
Diesel trucks with selective catalytic reduction warm their DEF (reductant) so it does not freeze. The engine computer switches the heaters and watches each heater control circuit.
P20C1 means the C heater control circuit looks open, as if a wire or heater has broken contact. It is a sibling of P20B9 (the A heater circuit); GM pickups often store both. TroubleCodes.net lists wiring, the heater or the engine computer as the causes.
On 2020-2021 GM pickups, owners report a DEF light, check engine light and an emissions speed limit; one dealer reportedly found a melted DEF tank connector. Cold weather makes the problem more likely to show.
Most likely causes
Cause 1: Damaged or overheated DEF tank connector or wiring
Supported by vehicle data- Why it ranks here
- Two 2020 Silverado 1500 owners relay dealer scans with P20C1; one says the dealer found a melted DEF tank connector. TroubleCodes.net also lists wiring.
- What would change this
- Likelier on a GM pickup with P20B9 stored too, or with discoloured terminals at the DEF tank.
Cause 2: Failed DEF heater element
Common general cause- Why it ranks here
- TroubleCodes.net lists the reductant heater, and an owner reports the code read as reductant heater failure.
- What would change this
- Likelier if the connector and wiring test good.
Cause 3: Engine computer or control module fault
Possible — needs testing- Why it ranks here
- TroubleCodes.net lists the ECM; nothing in the cases read ties it to a failure.
- What would change this
- Consider only after wiring and heater pass testing.
What to test first
Start at step 1. Each result tells you what it means and where to go next.
Step 1: Read all codes and note P20B9, P205D, P204C or DEF quality and level messages stored with P20C1
IfP20B9 stored too
It meansMore than one heater circuit is open; look at the shared DEF tank connector and harness.
Points to: Damaged or overheated DEF tank connector or wiring
IfOnly P20C1
It meansHave the C heater circuit tested.
Step 2: Have the DEF tank connector, terminals and heater circuit inspected and resistance-tested
Warning:A connector that has overheated can be a fire hazard; disconnect the battery first. DEF is a mild irritant and the tank area can be wet or frozen.
IfMelted or discoloured connector or open wiring
It meansRepair the harness or connector.
Points to: Damaged or overheated DEF tank connector or wiring
IfOpen heater element
It meansReplace the heater.
Symptoms
- Check engine light and DEF warning
- Speed limit or limp mode from the emissions system Gets more serious: stop and have it checked.
- More likely in cold weather
Compare with: Check engine light with loss of power
How to fix
On your vehicle
ChevroletYour car
2020–2021 · Silverado 1500
Supported by vehicle dataTwo 2020 owners report P20C1 with P20B9 and an emissions speed limit in extreme cold. No bulletin found in the cache.
GMCYour car
2021
Supported by vehicle dataA 2021 Sierra 3500 owner reports P20C1 with the DEF and check engine lights and a speed limit.
Related codes
Sources and review
- NHTSA complaint 11454519: 2021 GMC Sierra 3500
- NHTSA complaint 11710279: 2020 Chevrolet Silverado 1500
- NHTSA complaint 11704659: 2021 Chevrolet Silverado 2500
- NHTSA complaint 11710280: 2020 Chevrolet Silverado 1500
- P20C1 Trouble Code Definition
- Hino SB-18-013: COE heated DEF line DTCs P20BD, P20B9 or P20C1
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
- Pending
- Technician review
- Technician review pending
This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error