P21C4
Moderate Severity (Level 3)Reductant Heater Relay Control Circuit High
OBD-II diagnostic trouble code P21C4indicates a fault detected by the vehicle's onboard computer. This code applies to all OBD-II compliant vehicles and should be addressed promptly to avoid potential engine or emissions system damage.
Severity Level 3 — Moderate
This code indicates a moderate issue that may affect vehicle performance and fuel economy. Continued driving is possible in most cases, but prompt diagnosis and repair are recommended to prevent further damage.
Common Symptoms
- Check engine light on
- Reduced fuel economy
- Possible rough idle or hesitation
Possible Causes
- Reductant Heater Relay Control Circuit High
- Manufacturer-specific condition
- Fault in related component or circuit
Common Fixes
- Diagnose with professional OBD2 scan tool
- Check manufacturer TSBs
- Inspect related components per service manual
Estimated Repair Cost
$150 – $600(typical range; varies by vehicle make and model)
These are estimated costs based on generic repair data. Actual costs may vary depending on your vehicle, location, and labor rates.
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