P221F: Reductant Heater C Current Too High

What does P221F mean?

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.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

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

Common causes

• High Partly shorted heater C element
• Medium Heater lead chafed to ground at the tank or pump unit
• Low Water or DEF crystals in the heater connector

Likelihood is a general estimate across vehicles; your make and model may differ.