P21C3: Reductant Heater Relay Control Circuit Low

Severity

Medium

System

Powertrain

Last reviewed: Jan 1, 0001 Last updated: Jan 1, 0001

Methodology: We combine code standards with complaint and reliability patterns, then review for clear, vehicle-first diagnosis steps.

Sources: SAE/OBD-II definitions, manufacturer service information, and public reliability reports (including NHTSA where applicable).

Limits: Guidance is informational and may not match every model-year or revision. Confirm with the official manual and diagnostics before replacing parts. For safety-critical or electrical work, use a qualified technician.

What Does P21C3 Mean?

P21C3 is a diagnostic trouble code indicating: Reductant Heater Relay Control Circuit 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 P21C3

⚠️ Check Engine Light (MIL) illuminated
⚠️ Reductant Heater Relay reading at zero or minimum value
⚠️ Related system not functioning or in fail-safe mode
⚠️ Reduced engine performance

Common Causes

🔍 Open or broken wire in Reductant Heater Relay circuit
🔍 Poor ground connection at Reductant Heater Relay
🔍 Faulty Reductant Heater Relay with no output signal
🔍 Disconnected or loose Reductant Heater Relay connector
🔍 Blown fuse in related circuit

Frequently Asked Questions

What does P21C3 mean?

P21C3 indicates: Reductant Heater Relay Control Circuit Low. The vehicle's computer has detected that the Reductant Heater Relay is not operating within its expected parameters and stored this diagnostic trouble code.

Can I drive with code P21C3?

You can drive short distances for essential trips, but schedule a diagnosis soon. Driving with P21C3 for extended periods may cause additional problems.

How much does it cost to fix P21C3?

Repair costs for P21C3 typically range from $20 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

🔧 Repair broken wire or restore ground in Reductant Heater Relay circuit
🔧 Clean and secure Reductant Heater Relay connector and ground points
🔧 Replace failed Reductant Heater Relay

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.