P221E: Reductant Heater C Current Too Low

What does P221E mean?

P221E says DEF heater C pulls too little current while commanded on. Which element is C depends on the system. In GM's 2025 Duramax tables heater 3 is a second tank heater; the code needs a PWM command above zero and trips under 0.75 A after 40 readings taken every 0.1 s, a Type A fault with the MIL after one trip. GM's 2014 tables for the 6.6L LML gave the letter to the heater in the supply (pump) module instead: conductance of 0.14 S or less, checked once per trip with the battery at 11 V or more, Type B. Ford's 2023 diesel summary has a heater C only with a supply voltage check (P20C2) and does not list P221E. In frost the thawing of DEF around that heater is slow. P221F is the high-current partner, P20C1 the open control circuit.

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

Symptoms

• Malfunction indicator lamp on
• Slow DEF thawing in cold weather, dosing starts late

Common causes

• High Heater C element open or burnt (tank or supply module, by system)
• Medium Corroded or wet connector at the tank or pump unit
• Low Heater supply missing (fuse, relay, harness)

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