How to Fix the P21DD Code on Duramax (Pro-Level Solutions)

Using the Diagnostic Trouble Codes makes it easy to locate the exact issue in the engine or other components activated by a sensor. You can filter out all the side problems to diagnose and fix the problem with these codes.

The P21DD trouble code on Duramax means “Low Current in the Reduction tank Heater.” It mostly happens due to a defect in the reduction tank heater used to heat the DEF.

Besides, some other minor electrical component failure can trigger this issue closely connected to the DEF heating system.

This issue has simple solutions and can be repaired in the home garage only if you know the symptoms and fixes. You need to read this article completely to learn about DTC P21DD causes and fixes.

P21DD Code on Duramax

What Does P21DD Code Mean On Duramax?

Suppose the Reduction Heating system fails due to receiving a low current to send feedback through the sensor and control the heater effectively. Then the P21DD Diagnostic Trouble Code takes place through the Check Engine Light.

  • P = Powertrain, the system of components that propels your car forward
  • 2 = Standard Manufacturer Code
  • 1 = Fuel And Air Monitoring System
  • DD = Specific fault index (In our case, this denotes low voltage in the control module of the reduction tank heater)

Diesel Exhaust Fluid (DEF) monitors and controls the toxic contamination in the diesel engine’s exhaust.

To refrain DEF from freezing in cold regions, a small heater is used. And damage to this heater can cause very smoky exhaust, which is against the Federal law regarding environmental contamination.

How Serious Is P21DD Code on Duramax?

The seriousness of this issue lies in both truck health and breaking the strict federal emission standard of the USA. Driving your Duramax can even stall your car on a busy road.

If the reduction tank heater does not work properly, the DEF reservoir will experience gradual freezing. This reduces the rate of DEF participation with a catalytic converter to clean exhaust as much as possible before releasing them in atmosphere.

You can get charged by the cops to drive a car with a Check Engine Code issue and also for polluting the atmosphere.

What Causes P21DD On Duramax?

The P21DD Diagnostic Trouble can get set in your Duramax due to many major and minor failures. It is incorporated with the Fuel and Airflow monitoring system responsible for lowering nitrogen toxicity. Some potential causes for the P21DD are stated below:

  • Defect in the reduction tank heat
  • Defect in the control module of the heater
  • Broken wires of the control module connecting with the PCM
  • Damage to the SCR controller or PCM programming

How To Fix P21DD Code On Duramax?

The first and foremost step for any diagnosis is to use a calibrated OBD-II scanner to confirm the problem. It is done because a similar type of side issue can also show the symptoms of P21DD.

Step 1: Running OBD-II Scanner

Connect the OBD-II device, laptop, or tablet to your Duramax using the pin connector below the dashboard. Run the error issue scanning mode and wait for the verdict.

You need to get the Freeze frame Data which shows the vehicle’s condition when the code was set. It is useful information for further fixation.

Step 2: Taking Out The DEF Reservoir

The reduction of heat tank is installed within the Diesel exhaust Fluid reservoir. Hence you need to remove the entire reservoir. Release the reservoir clamps using the 13mm sockets.

Many inlet hoses and electrical wirings connect to the injector and PCM. Remove the 24-pin connector, pressure hose, breather hose, and inlet hose carefully.

Finally, secure the openings of the hoses with some wrapping to avoid dust and debris from contaminating the fluid inside the reservoir.

Step 3: Inspect The Reduction Tank Heater Control Module

Sometimes a fault or defect in the control module of the tank triggers the P21DD. Due to the lack of signal sent from the sensor to the Power Control Module, it happens.

This control module is located just above the heater tank. You need to remove three Allen bolts. Cut that zip ties loose and free the wires connecting to the heater.

Perform a visual inspection for physical damage. You can also use the voltmeter to check the current flow through the control module. If you find any damage, then the entire control module of the reduction tank heater requires a replacement.

Step 4: Replacing The Reduction Tank Heater

If the control module is working fine, the problem lies within the Heat tank itself. Get a new heat tank compatible with the Duramax model. It is better to check the model beforehand.

Turn the securing ring counterclockwise to remove the top. The rubberized sealing should be pulled off before removing the heater. Take out the heater and rotate the tank similarly to disengage the locking position.

You can install a new reduction tank heater in the same way. Ensure no dirt or debris falls on the fluid while making a replacement.

Step 5: Problem In The PCM

If none of the solutions work properly, you need to confirm that the PCM receives adequate signals from the control modules. Sometimes, the PCM may require calibration and reprogramming to solve the DTC issues.

Other Symptoms Of Getting a P21DD Code On Duramax?

One of the easiest ways to confirm the DTC P21DD is to examine the exhaust gas. The number of Nitrogenous compounds is increased when the reduction tank heater fails to work properly.

Moreover, some other basic symptoms are there using which, you can be assured that something in your Duramax is causing P21DD.

  • Check Engine Light remains constantly turned on
  • Reduction in fuel economy
  • Low acceleration and speed
  • High usage of fuel
  • Dense Black smoke from the exhaust pipe
  • Degrading engine performance
  • Other Selective Catalytic Reduction system issues

How Much Does it Cost to Fix Duramax P21DD Code?

The total cost of the fixing program of P21DD depends upon the product and service related to replacement, disassembling, and diagnosis.

To change the reduction tank heater system, you need to pay around $168 to $178. The price is variable and depends upon the marketplace and authorization of the place.

Some dealers may charge you $1100 with a complete P21DD solution and a 50000-mile warranty solution.

Can You Drive Your Duramax With Code P21DD?

Since the P21DD does not affect the engine ignition and combustion system, you can drive your Duramax. But it is not recommended to drive the truck because you may be hampering the atmosphere and the exhaust system, which may increase the expense.

Moreover, driving a vehicle with a Check Engine Light being turned on is illegal. Thus you can be charged for breaking the federal laws of environmental pollution and traffic rule-breaking.


It is a common issue to face a DTC P21DD after driving your Duramax more than 4000 to 50000 miles. The reduction tank heater gets damaged due to heavy usage.

Therefore you can use the fixes mentioned above to fix your truck and enjoy more smooth rides. With some common tools and understanding, you can only solve the issue in the home garage.

This way, you can save some extra bucks from the service charge workshop or dealers take. Lastly, the longevity of your Duramax Truck gets enhanced and becomes ready to serve you even for more years.

Similar Posts