How to Fix P20b9 Code On Duramax? (Completely Explained)

The code P20b9 on Duramax is a bit intimidating, especially if you’re reading it for the first time. No worries, the first thing you should look for is a clear understanding of what the code means on Duramax.

In technical terms, the code P20b9 stands for “Reductant Heater “A” Control Circuit/Open”. In other words, when the powertrain control module detects no voltage in the control circuit of the reductant heater, it sets the code.

The next step is to figure out how to fix the code once you’ve figured out what it means. In this article, I’ll discuss how to correct the code.  I’ll also address the problems, causes, and potential threats of the code.

P20b9 Code On Duramax?

What Does Code P20b9 Mean for Duramax?

As previously stated, in a vehicle with a Duramax engine, the code P20b9 stands for “Reductant Heater” Control Circuit/Open.” In other words, when the powertrain control module detects no voltage in the control circuit of the reductant heater it sets the code.

It happens when the reductant temperature sensor of the Duramax engine fails to detect the extreme temperature of the diesel fluid exhaust, the PCM detects no voltage.

Voltage is required by the glow plug to heat the heating element. The code is set as soon as the PCM detects no voltage. To give you a better understanding of the code. Here is a break of the code P20b9.

  • P denotes the vehicle’s powertrain.
  • 2 indicates a generic number derived from the SAE standard.
  • 0 refers to the Reductant Heater.
  • B9 indicates a faulty reductant heater circuit open

How serious is the code P20B9 on Duramax?

P20b9 on Duramax is a serious code and must be taken seriously and corrected as soon as possible. Here is an explanation.

Selective catalytic reduction (SCR) systems are used in modern, powerful diesel engines like the Duramax to convert exhaust particles into harmless gases. To accomplish this, the system’s temperature must rise.

The PCM monitors the SCR systems. If the PCM does not receive voltage, it cannot determine the proper timing for DEF injection, resulting in additional engine issues.

There are additional consequences too. The SCR system may be disabled if the code is not fixed as soon as possible. In addition, the vehicle’s reducing catalyst will be damaged, resulting in the code being stored.

What are the causes of Code P20b9?

In Duramax, the faulty reductant heater is the most obvious cause of the code.  The following are some other possible causes of the code:

  • Shorted or opening in the circuit
  • Loose electrical connections
  • damaged wiring
  • The reductive heater control is faulty.
  • The issue with the reductant tank heater
  • Problems in the SCR or PCM controller
  • There might be a programming error.
  • A defective temperature sensor

When fixing the code,  addressing and resolving these causes will almost certainly result in the code being removed.

What are the symptoms of Code P20b9 on Duramax?

Like other DTC codes, code P20b9 can have symptoms. Here are some of the most common P20b9 symptoms.

  • Poor engine performance
  • Check the engine light
  • Exhaust releases more black smoke.
  • There might be other SCR related codes.
  • Fuel efficiency gets reduced.
  • It’s possible that DPF won’t regenerate.

Diagnosis and Fixation of the Code P20b9

When a code appears, it indicates that your vehicle has a specific problem within a particular area. However, it remains unclear as to what is causing the problem. That is why. After the code, you’ll need to diagnose the system to determine the specific reason.


You’ll need a vehicle-specific repair guide for the Duramax that matches the make, year, and engine size before you begin the diagnosis. You can get a step-by-step process, a wiring diagram, and other explanations from the guides.

Basic Tools Required:

The OBD Scanner to interpret the OBD code

A digital Volt or ohmmeter to check the resistance or volt

Start Diagnosis

This step-by-step guide is provided for educational purposes only and is not intended to be a professional recommendation. I would recommend finding the most experienced professional available to repair the code.

Step One: Visual Inspection

Begin by performing a visual inspection. Locate the faulty wire and its wiring connector, if any. Repair all damaged wiring and connectors before moving on to the next step.

After that, connect the scanner to the vehicle’s OBD port to obtain all of the stored codes and other relevant information. Before moving on to the next step, clear this code and all the freeze-frame data.

Step Two: Check the Voltage

At this stage, use the ohmmeter to see if the SCR control system has a steady power supply or if the voltage is maintained. Check the fuses as well to avoid a misdiagnosis. You may need to reprogramme or recalibrate if the voltage does not fall within the specified range.

Step Three: Check the Voltage

If the voltage is correct and within the range specified by the vehicle, the next step is to test the reductant heater and ensure that the test result is within the expected parameter. If this is not the case, take the necessary action.

Specific parts that may need to be replaced to fix the code

Some parts that may directly contribute to the problem should be replaced as soon as possible. Here is a list of them:

Repare or replace heater: The circuit inside the reductant heater could be shorted high or low, causing it to fail to perform the required task. In this case, the reductant heater may need to be replaced.

Repare Temperature Sensor: If the temperature sensor is defective, it will not send the necessary data to the PCM. As a result, it must be replaced.

Clean or Replace DEF (Diesel Exhaust Fuel): DEF stands for Diesel Exhaust Fuel. The DEF becomes dirty over time, contributing to the problem. DEF must be replaced as soon as possible to eliminate the problem.

Damaged wires and connectors: They can significantly contribute to the P20b9 code problem and should be repaired or replaced as soon as possible.

The Cost of Fixing The Code P20B9

Several factors determine the cost of repairing the code. If you decide to do it yourself, you will need essential repair tools such as a scanner or an Ohmmeter, ranging from $200 to $300.

If you hire a professional to complete the same task, you should expect to pay between $150 and $200.

When it comes to the cost of specific replacement parts such as the temperature sensor or the DEF, you have to bear them. You will also have to pay for hidden fees, such as tips.

Can you drive with code P20B9?

Earlier, we have stated the severity of the code. The code P20B9 should be considered severe. SCR is an important part and can be disabled. Catalyst damage can also be the result. So with such a threat, I think you should not drive the car, and the issue has to be solved as soon as possible.


The code P20b9 for Duramax, like other codes, is a common occurrence for owners. The code should no longer puzzle you as you now know everything there is to know about it.

To figure out how serious a code is, start by asking what the code means. Find out the reasons for the code. Consult with the technician as soon as you have the reason. I believe the article was informative, and you were successful in resolving the Duramax P20b9 error code.

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