BMW DTC P21CB – Reductant Control Module Supply Voltage Low
DTC P21CB meaning on BMW
The Diagnostic Trouble Code (DTC) P21CB for BMW indicates a fault related to the Reductant Control Module Supply Voltage being low. The reductant control module is responsible for managing the supply of the reducing agent (commonly known as AdBlue or DEF) in vehicles equipped with Selective Catalytic Reduction (SCR) systems to reduce emissions.
BMW DTC P21CB symptoms
Symptoms of the DTC P21CB on a BMW may include:
- Check Engine Light (MIL) illuminated
- Reduced engine performance
- Possible DEF system warnings
BMW DTC P21CB causes
Common causes of the DTC P21CB on a BMW include:
- Defective reductant control module
- Wiring or connection issues in the reductant control module circuit
- Low voltage supply to the reductant control module
BMW DTC P21CB seriousness
The DTC P21CB should be addressed promptly as it can lead to increased emissions and potential non-compliance with emission regulations. Ignoring this fault may result in reduced engine efficiency and possible damage to the SCR system components.
How to diagnose DTC P21CB on BMW
To diagnose the DTC P21CB on a BMW, a mechanic would typically perform the following steps:
- Use a diagnostic scanner to retrieve the fault codes and freeze frame data
- Inspect the reductant control module and its wiring for any visible damage or corrosion
- Check the voltage supply to the reductant control module using a multimeter
- Perform a thorough inspection of the DEF system components for any leaks or malfunctions
How to fix DTC P21CB on BMW
To fix the DTC P21CB on a BMW, the following steps may be taken:
- Replace the faulty reductant control module if diagnosed as the root cause
- Repair any damaged wiring or connections in the reductant control module circuit
- Ensure proper voltage supply to the reductant control module
- Clear the fault codes and perform a test drive to verify the repair
How to erase DTC P21CB on BMW
To erase the DTC P21CB on a BMW, the fault codes can be cleared using a diagnostic scanner. After the repair has been completed and the underlying issue addressed, the codes can be reset to turn off the Check Engine Light and ensure the system is functioning properly.
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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