P21CB Code - Reductant Control Module Supply Voltage Low Voltage
Quick Answer
The P21CB code indicates that the Reductant Control Module (RCM) is experiencing low voltage. Common causes include Faulty Power Supply: A faulty power supply to the Reductant Control Module can result in low voltage and trigger this code, Wiring Issues: Damaged or corroded wiring connected to the RCM can cause a drop in voltage supply, Malfunctioning Reductant Control Module: The RCM itself may be malfunctioning, leading to low voltage readings. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.
- What it means
- P21CB indicates Reductant Control Module Supply Voltage Low Voltage.
- Common causes
- Common causes include Faulty Power Supply: A faulty power supply to the Reductant Control Module can result in low voltage and trigger this code, Wiring Issues: Damaged or corroded wiring connected to the RCM can cause a drop in voltage supply, Malfunctioning Reductant Control Module: The RCM itself may be malfunctioning, leading to low voltage readings.
- Severity
- High - diagnose this code as soon as possible.
- Can I drive?
- Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.
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Code P21CB Description
The P21CB code indicates that the Reductant Control Module (RCM) is experiencing low voltage. The Reductant Control Module is responsible for controlling the injection of diesel exhaust fluid (DEF) into the exhaust stream to reduce emissions. When the voltage supplied to the RCM is low, it can affect the proper functioning of the DEF system, leading to increased emissions and potential engine performance issues.
Common Causes of P21CB
- Faulty Power Supply: A faulty power supply to the Reductant Control Module can result in low voltage and trigger this code.
- Wiring Issues: Damaged or corroded wiring connected to the RCM can cause a drop in voltage supply.
- Malfunctioning Reductant Control Module: The RCM itself may be malfunctioning, leading to low voltage readings.
- Battery Issues: A weak or failing battery can also cause low voltage supply to the RCM.
Symptoms of P21CB
- Check Engine Light: The most common symptom associated with this code is the illumination of the check engine light on the vehicle's dashboard.
- Reduced Engine Performance: The vehicle may experience a decrease in engine performance, such as reduced power or acceleration.
- Increased Emissions: The vehicle may produce higher emissions than normal due to the improper functioning of the DEF system.
- Poor Fuel Efficiency: A decrease in fuel efficiency may be noticed as a result of the low voltage supply to the RCM.
How to Fix P21CB
- Diagnostic Scan: Use an OBD-II scanner to retrieve the specific fault code and pinpoint the issue.
- Check Power Supply: Inspect the power supply to the Reductant Control Module to ensure it is functioning properly.
- Check Wiring: Inspect the wiring connected to the RCM for any damage or corrosion and repair or replace as needed.
- Replace Reductant Control Module: If the RCM is found to be faulty, it may need to be replaced to resolve the low voltage issue.
- Clear Codes: After repairing the issue, clear the fault codes and reset the check engine light to ensure the problem has been resolved.
Cost to Fix P21CB
The cost of repairing a P21CB code can vary depending on the underlying cause of the issue. In general, the cost of parts for this repair can range from $100 to $500, while labor costs can range from $100 to $300. Overall, the total repair cost may fall between $200 and $800. However, it is important to note that labor rates at auto repair shops can vary, so it is advisable to check with local shops for a more accurate estimate based on your specific vehicle make and model.
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Code P21CB Information for Specific Makes
- P21CB Buick — Reductant Control Module Supply Voltage Low Voltage
- P21CB Cadillac — Reductant Control Module Supply Voltage Low Voltage
- P21CB Chevrolet — Reductant Control Module Supply Voltage Low Voltage
- P21CB GMC — Reductant Control Module Supply Voltage Low Voltage
Frequently Asked Questions about P21CB Code
1. What does the OBDII code P21CB mean?
P21CB indicates that the Reductant Control Module is experiencing a low supply voltage, which may affect the diesel exhaust fluid (DEF) system's operation.
2. What are the common symptoms of the P21CB code?
Symptoms may include an illuminated check engine light, DEF system warnings, reduced engine power, or failure to meet emission standards.
3. What causes the P21CB code to appear?
Common causes include a faulty reductant control module, corroded or damaged wiring, a weak battery, or a malfunctioning alternator.
4. How can I diagnose the P21CB code?
Diagnosis typically involves checking the voltage supply to the reductant control module, inspecting wiring and connectors for damage, and testing the battery and charging system.
5. Can I drive with the P21CB code?
Driving with the P21CB code is not recommended, as it may lead to reduced engine performance and potential non-compliance with emissions regulations.
6. How do I fix the P21CB code?
Repairs may include replacing the reductant control module, repairing damaged wiring, cleaning corroded connections, or addressing issues with the battery or alternator.
7. How much does it cost to fix the P21CB code?
Repair costs vary but can range from $100 for wiring repairs to $500 or more for module replacement, depending on the vehicle and labor rates.
8. Can a dead battery trigger the P21CB code?
Yes, a dead or weak battery can cause low voltage to the reductant control module, triggering the P21CB code.
9. Is the P21CB code specific to certain vehicles?
The P21CB code is most commonly found in diesel vehicles equipped with a DEF system, such as trucks and SUVs.
10. Will clearing the P21CB code solve the problem?
Clearing the code may temporarily turn off the check engine light, but the underlying issue must be repaired to prevent the code from returning.
Discuss the causes and fixes for P21CB OBDII Code
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Hello, I have an error code: P21CB on an Izuzu D-max 2021 v cross engine 1.9l diesel, the problem occurred when the car would not start due to the battery. When trying to start the car, the DPF light first came on, then the yellow "check engine" light. I started the car with the new battery, the lights went off by themselves but the errors remained stored. In Europe, Izuzu dealerships are a bad joke, which I can check myself. Thank you very much for your patience.
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Thanks for the detailed explanation — you’re already on the right track!
The P21CB code stands for "Reductant Control Module Supply Voltage Low", which refers to low voltage detected by the DEF (Diesel Exhaust Fluid) system's control module. This system is part of your DPF and emissions control setup.
Based on your description:
- The issue happened after a weak battery or failed start
- You replaced the battery, and now the car starts and runs fine
- The DPF and check engine lights went off, but the code remains stored
This suggests the code was triggered due to low system voltage during cranking—a very common cause for this type of fault.
What You Can Do:
Clear the Code Manually
Since the system is working fine now and the lights went off:- Use an OBD2 scan tool to clear the stored code (you can often borrow one from a parts store or buy a basic tool online).
- Once cleared, monitor to see if it returns.
Check Battery & Charging System
Even with a new battery, make sure the:- Battery voltage is 12.6V+ when off and 13.8–14.5V while running
- Terminals are clean and tight
- Alternator is charging correctly
Inspect Fuses & Wiring to the Reductant Control Module
If the code returns:- Look for any blown fuses or loose/corroded wiring near the DEF system or battery.
- The Reductant Control Module is often located near the DEF tank or under the vehicle, depending on the model.
Summary:
- The code was likely a result of low voltage during starting attempts, not an actual failure.
- If the vehicle is running normally and the warning lights are off, it's safe to clear the code and move on.
- If the code comes back, then dig deeper into the DEF system wiring and power supply.
Let us know if you need help locating the module or clearing the code!
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Hello, thank you very much for your prompt and explicit response. I would be happy if you could tell me where the control module is located.
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You're very welcome! I'm glad the info helped!
On the 2021 Isuzu D-Max 1.9L diesel, the Reductant Control Module (which manages the DEF/AdBlue system) is typically located under the vehicle, near or attached to the DEF (AdBlue) tank, which is usually on the passenger side (right-hand side) under the rear portion of the chassis.
To locate it:
- Go underneath the vehicle, near the DEF tank — it’s a small tank separate from the diesel fuel tank.
- Look for a small electronic module with wiring connected to it, often mounted directly on the DEF tank or close by.
- You may also see the DEF pump and heater unit, which are part of the same system and often integrated into the module housing.
Pro Tip:
- The module is usually exposed to the elements, so check for any corrosion, damaged wires, or loose connectors.
- If you have access to a lift or ramps, it’ll make inspection a lot easier and safer.
Let me know if you'd like a step-by-step guide to test or inspect it!
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