P21C4 — Reductant Heater Relay Control Circuit High

What Does P21C4 Mean?

P21C4 is a diagnostic trouble code indicating: Reductant Heater Relay Control Circuit High. This code relates to the powertrain system. The vehicle's computer detected a condition outside normal operating parameters and stored this code.

Symptoms of P21C4

⚠️ Check Engine Light (MIL) illuminated
⚠️ Reductant Heater Relay reading stuck at maximum value
⚠️ Related system operating erratically or in default mode
⚠️ Reduced engine performance

Common Causes

🔍 Short to voltage in Reductant Heater Relay wiring harness
🔍 Corroded or water-damaged Reductant Heater Relay connector
🔍 Faulty Reductant Heater Relay outputting maximum voltage
🔍 Damaged wiring rubbing against metal components
🔍 ECM/PCM internal fault (rare)

Frequently Asked Questions

What does P21C4 mean?

P21C4 indicates: Reductant Heater Relay Control Circuit High. 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 P21C4?

You can drive short distances for essential trips, but schedule a diagnosis soon. Driving with P21C4 for extended periods may cause additional problems.

How much does it cost to fix P21C4?

Repair costs for P21C4 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

🔧 Inspect and repair Reductant Heater Relay wiring for short to voltage
🔧 Clean or replace corroded Reductant Heater Relay connector
🔧 Replace faulty 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.

P21C4: Reductant Heater Relay Control Circuit High