P20C1: Reductant Heater C Control Circuit/Open

Heater C's control circuit is interrupted. Where the third element sits again depends on the maker: GM's 2025 DEF tables make heater 3 a second tank heater and test it like the others (output off, 2 s, Type A); Deutz assigns C to the return line heater; FCA's EcoDiesel has a heater 3 of its own. Hino's 2018 bulletin for J05 cab-overs lists P20C1 as heater 3 of the heated DEF line, specified at 5.5 ohms ±1 at 20 °C and replaced as a complete line if out of range. On the 6.6 L LML in GM's 2014 tables the glow plug control module drives the DEF heaters and reports this fault after an active test that discharges its capacitor, about 3.4 s, Type B. The affected section stays frozen longer after a cold start, delaying dosing.

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

Warning lamp mainly after cold soaks
DEF dosing delayed after a cold start
SCR warning and countdown if the fault persists

Most likely causes

Heating circuit in the line or tank module open (resistance check at the connector)
Unplugged or corroded heater C connector
Glow plug control module fault where it drives the DEF heaters (GM LML)

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
Check the continuity of the wire between the component and the control module.
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 P20C1 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
0: Subsystem depends on the code range
C1: Specific fault in that system

Frequently asked questions

Can I keep driving with code P20C1?

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

What causes code P20C1?

The most common causes are: heating circuit in the line or tank module open (resistance check at the connector); unplugged or corroded heater C connector; glow plug control module fault where it drives the DEF heaters (GM LML).

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.