P21C9
Moderate Severity (Level 3)Reductant Control Module Power Relay/Relays Control Circuit High
OBD-II diagnostic trouble code P21C9indicates 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 Control Module Power Relay/Relays 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.
Explore Repair Costs
Worried about repair costs?
Get transparent estimates and verify your mechanic's quote before authorizing repairs.
Related Discussions
See what other car owners are saying about this code and related repairs.
View P21C9 discussions in the community