P21C4

Reductant Heater Relay Control Circuit High

Severity

⚠️ Medium

Can you drive?

Yes, but get it checked soon

Approx. repair cost

$17–89 (est.)

Symptoms

Check Engine Light Is On
Possible reduction in engine power
Increased NOx emissions
SCR error message on dashboard
Problems starting in cold weather

Causes

Short circuit to positive in the relay control circuit
Reagent heater relay malfunction
Wiring harness damage
Problems with the engine control unit
Corrosion of contacts or connectors

How to Fix

Check relay control circuit for short circuit
Test the reagent heater relay
Inspect the wiring harness for damage
Check contacts and connectors for corrosion
If necessary, replace the relay or repair the wiring

Related codes

P21C3 P21C5 P20EE