P21C4 – BMW DTC

BMW DTC P21C4 – Reductant Heater Relay Control Circuit High

DTC P21C4 meaning on BMW

The DTC P21C4 code for BMW refers to an issue with the reductant heater relay control circuit being detected as high. This means that there is a problem with the control circuit that manages the reductant heater relay, which is responsible for heating the diesel exhaust fluid (DEF) to optimize its performance in reducing emissions.

BMW DTC P21C4 symptoms

Common symptoms associated with the DTC P21C4 code on a BMW may include:

• Check Engine Light illuminated
• Reduced engine performance
• Possible DEF system warnings

BMW DTC P21C4 causes

The potential causes of the DTC P21C4 code on a BMW can include:

• Faulty reductant heater relay
• Wiring issues in the relay control circuit
• Defective temperature sensors

BMW DTC P21C4 seriousness

While the DTC P21C4 code may not cause immediate drivability issues, it is important to address it promptly to ensure proper emission control and overall vehicle performance. Ignoring this code can lead to increased emissions and potential long-term damage to the vehicle’s emission control system.

How to diagnose DTC P21C4 on BMW

Diagnosing the DTC P21C4 code on a BMW typically involves:

1. Using a diagnostic scanner to retrieve the fault codes
2. Inspecting the reductant heater relay and its wiring for any visible damage
3. Testing the relay and related components for proper functionality

How to fix DTC P21C4 on BMW

To fix the DTC P21C4 code on a BMW, you may need to:

1. Replace the faulty reductant heater relay
2. Repair any damaged wiring in the relay control circuit
3. Check and replace defective temperature sensors if necessary

How to erase DTC P21C4 on BMW

After addressing the underlying issue causing the DTC P21C4 code on a BMW, you can erase the code using a diagnostic scanner. This will reset the Check Engine Light and clear the stored fault code from the vehicle’s computer system.

Recommended tools & DTC Scanners for BMW

Best Diagnostic App for modern BMWs: BimmerLink

Recommended Bluetooth adapter for Bimmerlink: OBDLink CX

Cable adapter for Bimmerlink: Enet Cable USB Type C

For older BMWs we recommend: Autel AutoLink AL319 (2024 version)

Easy-to-use Multimeter for reliable diagnostics: AstroAI Auto-ranging Multimeter

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