P221C: Reductant Heater B Current Too Low

What does P221C mean?

P221C reports that heater B of the DEF system draws too little current while switched on. GM and Ford both assign B to the heated pressure line that carries DEF to the dosing valve. GM's 2025 Duramax tables flag line heater current under 0.75 A with a PWM command above zero, 40 faulty readings spaced 0.1 s apart, Type A with the MIL after one trip. Ford's 2023 diesel summary watches heater B continuously while it runs and decides within 2.2 s: below 3 A with more than 5 V supplied, line power below 1 W or lower than expected. GM's 2014 tables for the 6.6L LML judged conductance instead (0.28 S or less), Type B. An open or burnt line heater is the usual find; P221D is the high-current partner.

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

Symptoms

• Malfunction indicator lamp on
• DEF line may freeze in winter, dosing starts late
• DEF system warning, inducement possible if unresolved

Common causes

• High Heating element in the DEF pressure line open or burnt
• Medium Corroded or loose connector at the heated line
• Low Missing heater supply (fuse, relay or harness)

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