Powertrain, generic (extended)

P221F Reductant Heater C Current Too High

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

P221F flags excessive current through DEF heater C, pointing to a partial short in the element or its lead. GM's 2025 Duramax tables, where heater 3 is the second tank heater, set the code above 15 A over 40 samples of 0.1 s whenever the vehicle is active, Type A with the MIL after one trip, and zero all heater PWM commands while any heater fault is present. In GM's 2014 tables for the 6.6L LML, C denoted the supply module heater, and high conductance (0.46 S or more in one table, 0.35 S in the other) set a Type B fault once per trip. Ford's 2023 diesel summary does not use P221F. A shorted heater draws more than its driver tolerates, so the area it serves stays unheated. P221E is the low-current partner.

Likely causes

Partly shorted heater C element

Heater lead chafed to ground at the tank or pump unit

Water or DEF crystals in the heater connector

Symptoms

Malfunction indicator lamp on after one trip (GM)
DEF heating disabled, possible frozen-DEF warnings in winter

Repair estimate

DifficultyHardWorkshop job

Parts and labour€100–1,800

Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

reductant line heater c

References: gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com, law.cornell.edu.