P20C3: Reductant Heater C Control Circuit Low

A low C circuit points to heater C being grounded somewhere or drawing excessive current. GM's 2025 DEF module treats heater 3, its second tank heater, exactly like heaters 1 and 2: a low-band voltage with the output off, or drain voltage and current limits with it on, Type A once 2 s have elapsed. On the 2014 LML Duramax (6.6 L) the glow plug control module reports heater currents above 25 A within about 1 s (Type B). The best-documented case is GM bulletin 22-NA-112: on 2020–2022 Silverado and Sierra with the 3.0 L LM2 or 6.6 L L5P, P20BB and P20C3 could set without any defect, and a recalibration of the DEF control module changed the test. Deutz maps the code to its return line heater (FMI 4). On those trucks software comes first; elsewhere look for a chafed harness.

Medium severity. Can I keep driving?Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

Symptoms

Malfunction indicator lamp, possibly with no real fault present
Driver message that speed will be reduced without service
Unheated tank or line section in frost

Most likely causes

Outdated DEF control module calibration on 2020–2022 GM diesel pickups (22-NA-112)
Heater C wire grounded through chafing
Shorted heater C element drawing overcurrent

What to check, in order

Start with the simplest. You can tick off each step.

Check the AdBlue/DEF level and quality, and do not add additives or water
With the connector unplugged, check whether the signal wire is making contact with ground.
Look for white crystals around the injector and the tank
Check connectors and heaters with the scanner
Run the dosing test available on your diagnostic tool
Replace parts only when the test points to that component

Good to know

Repair difficulty: medium
It can trigger limp mode.
It affects emissions.

What each character of P20C3 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
0: Subsystem depends on the code range
C3: Specific fault in that system

Frequently asked questions

Can I keep driving with code P20C3?

Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

What causes code P20C3?

The most common causes are: outdated DEF control module calibration on 2020–2022 GM diesel pickups (22-NA-112); heater C wire grounded through chafing; shorted heater C element drawing overcurrent.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.