Powertrain, generic (extended)

P221C Reductant Heater B Current Too Low

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

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

Symptoms

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

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€100–700
Time0.5–2.5 h

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

Parts involved

reductant line heater b

Related codes

P221DP20BDP20BEP20BFP20C0P21DDP221E

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