P21CB Code - Reductant Control Module Supply Voltage Low Voltage
Quick Answer
The selective catalytic reduction system reduces Oxides of Nitrogen (NOx) emissions by injecting a metered amount of reductant into the exhaust gas stream entering the selective catalytic reduction catalyst. Common causes include Faulty Reductant Control Module, Reductant Control Module harness is open or shorted, Reductant Control Module circuit poor electrical connection. 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 Reductant Control Module, Reductant Control Module harness is open or shorted, Reductant Control Module circuit poor electrical connection.
- 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.
Code P21CB Description
The Diagnostic Trouble Code (DTC) will be set when the Reductant Control Module supply voltage is less than a calibrated value for greater than 5 s.
What are the Possible Causes of the DTC P21CB?
- Faulty Reductant Control Module
- Reductant Control Module harness is open or shorted
- Reductant Control Module circuit poor electrical connection
How to Fix the DTC P21CB?
Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.
What is the Cost to Diagnose the Code?
Labor: 1.0
To diagnose the P21CB code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.
Need more information about the P21CB code?
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Need Help with the Code?
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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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