Powertrain, generic (extended)

P221D Reductant Heater B Current Too High

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

P221D is stored when heater B, the electrically heated DEF pressure line at GM and Ford, takes more current or power than its design permits. That usually means a partly shorted element or a lead touching ground, and the controller cuts the heater to save its driver. GM's 2025 Duramax tables trip above 15 A, counted over 40 samples of 0.1 s while the vehicle is active, Type A with the MIL on the first trip and recovery only in the next drive cycle. Ford's 2023 diesel summary flags line heater power above the expected value within 2.2 s of continuous monitoring. On the 2014 6.6L LML, GM read the line heater conductance and set the code at 0.92 S or more, Type B. P221C is the low-current partner, P20BF a low control circuit.

Likely causes

Partly shorted heating element in the DEF pressure line

Heater lead chafed to ground

Water in the line heater connector

Symptoms

Malfunction indicator lamp on after one trip (GM)
Line heater switched off, DEF may stay frozen in cold weather

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

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