P21CB — Reductant Control Module Supply Voltage Low

What Does P21CB Mean?

P21CB is a diagnostic trouble code indicating: Reductant Control Module Supply Voltage Low. This code relates to the powertrain system. The vehicle's computer detected a condition outside normal operating parameters and stored this code.

Symptoms of P21CB

⚠️ Check Engine Light (MIL) illuminated
⚠️ Reductant Control Module Supply Voltage Low not functioning as expected
⚠️ Related system may be in fail-safe mode
⚠️ Reduced engine performance

Common Causes

🔍 Faulty Reductant Control Module Supply Voltage Low
🔍 Wiring or connector issue in Reductant Control Module Supply Voltage Low circuit
🔍 Related mechanical or electrical fault
🔍 ECM/PCM issue (rare)

Frequently Asked Questions

What does P21CB mean?

P21CB indicates: Reductant Control Module Supply Voltage Low. The vehicle's computer has detected that the Reductant Control Module Supply Voltage Low is not operating within its expected parameters and stored this diagnostic trouble code.

Can I drive with code P21CB?

You can drive short distances for essential trips, but schedule a diagnosis soon. Driving with P21CB for extended periods may cause additional problems.

How much does it cost to fix P21CB?

Repair costs for P21CB typically range from $80 to $300, depending on the root cause. Simple wiring or connector repairs are on the lower end, while component replacement costs more. A proper diagnosis ($80–$150 at most shops) will identify the exact cause before committing to expensive parts.

What To Do Next

Possible Fixes

🔧 Inspect Reductant Control Module Supply Voltage Low and related wiring
🔧 Replace faulty Reductant Control Module Supply Voltage Low

1
Find your vehicle above
Click your make and model for real owner reports and common causes specific to your vehicle.
2
Check for recalls
Go to nhtsa.gov/recalls and enter your VIN. Free repairs if covered.
3
Get a professional diagnosis
A code alone doesn't identify the exact failed part. A diagnostic ($50–$150) pinpoints the root cause.

P21CB: Reductant Control Module Supply Voltage Low