Last Updated: 05.11.2021

Are you having some problems with your transmission?

If your truck has TCM issues, it could lead to compromised driving performance and poor fuel economy. You could also experience unpredictable shifting, downshifting issues, check engine light, and trouble shifting to high gear.

All these issues could prompt you to change the transmission. But hold on a second… before changing the transmission, you should first test the TCM on Duramax to see what’s going on.

This is why it’s good to learn how to test TCM Duramax to help you pinpoint any problems and decide whether or not you need a TCM change.

Keep on reading for more details!

What Is a TCM in a Truck?

tcm on a truck above the alternator

Your Duramax’s TCM or Transmission Control Module is an extremely important part of the truck.

The first and foremost function of the TCM is to get vital information from sensors as well as drones at an actuator that engages gears. There are several sensors and switches linked to the transmission control module, including the valve body, power switch, overdrive switch, brake pedal switch, gear lever switch, torque converter clutch solenoid, and shift solenoid.

Valve Body

This is the control center of your truck’s automatic transmission. It normally bolts to the transmission case and is contained in the transmission pan. The valve body is mainly made up of valves, separator plates, and check balls.

Shift Solenoid

This is an electronically controlled switch valve that allows the fluid pass to actuate the actual gear shift. Then it determines the correct upshifts and downshifts points for all driving conditions.

Torque Converter Clutch Solenoid

The function of this solenoid is to move a valve that drives pressurized fluid to get to the clutch circuit to engage the TCC.

Gear Lever Switch

This switch gives the signal to which mode you should drive the truck.

Brake Pedal Switch

This is an on/off switch that is located in front of the brake pedal. It provides a signal when you press or release the brake. The transmission converter clutch, on the other hand, disengages when you brake and engages when you release the brake.

Overdrive Switch

This switch is operated by the driver to manually select or deselect the overdrive. But don’t use this feature when pulling a heavy load because doing so can overload the transmission.

Power Switch

The power switch is used in the programming of the automatic transmission. In the power mode, the TCM will hold gears for extended periods, and during takeoff and acceleration.

Symptoms of a Bad TCM Duramax

Here are the various symptoms you should look out for:

Shifting Problems

The sensors convey information to the Transmission Control Module and the TCM reads the information to establish the best time for the truck to shift gears. The TCM also stores information about the transmission. 

When the TCM goes bad, the transmission of your truck may start to shift erratically. The common shift issues are:

  • Transmission may not respond or get stuck in neutral
  • The transmission may begin to slip
  • The transmission may start to shift in the wrong gear
  • The transmission may not be able to downshift when the truck is approaching a stop light

Slow Response

When the TCM is failing, you may experience a significant delay in response. This usually happens when you’re shifting from park to drive or reverse. 

The issues associated with slow response include:

  • No response when stepping on the gas pedal
  • The truck may take a long time to accelerate as well as increase speed

Slipping Transmission

A bad Transmission Control Module could easily cause slipping transmission.

There are clutch bands in an automatic transmission that grip the different gear sets to transfer from the engine to the wheels. If these bands don’t have enough force to grip down tightly enough, they will slip and you will experience some issues.

But here is the thing, slipping could also be caused by failing torque converter, shift solenoid problems, low transmission fluid, worn-out gears, and worn clutch pack.

Transmission Codes that Will Not Go Away

If the Transmission Control Module is not working properly, it may trigger several codes that don’t go away. At this point, you will have to test and inspect the TCM and replace it if necessary.

Check Engine Light

Transmission issues could trigger the check engine light. But it is not as common as with other parts within your Duramax truck.

How to Test TCM Duramax

What You Will Need

Here are the things you will need to test TCM on Duramax:

  • Scanning Tool
  • Multimeter
  • Scope
  • TCM Software Transfer Tool
  • Safety Equipment
  • Battery Charger

Step-By-Step Instructions

To test the Duramax TCM, follow these step-by-step instructions:

Step 1: Manually Inspect the TCM

manually inspect the tcm

Inspect the TCM manually to see if the issue with the system is simple or complex. Check the TCM programming harness. Vibration and jerks of your Duramax can pull the pins past the lock. This leads to intermittent connection.

Step 2: Locate the OBD II Port

locate the obd II port

The OBD II port may vary in color depending on your truck, which is white or black. The most common location of the OBD II port is usually on the driver’s side of the truck. This is right underneath the steering.

Keep in mind that you might have to be outside the truck to get low enough to see the port. Your truck may also have a cover that you will have to remove to get to the port. You can also find it around or under the center console.

Step 3: Connect the Scanning Tool

connect the scanning tool

You can use a scanner of your choice. But you need a high-performing scanner that not only can read but also clear and reset fault codes.

The scanner should also be able to read live data. The device should be strong as well as reliable. The installation has to be easy and it should be able to find your truck automatically. The controls on the tool should be super simple to use.

If there’s a problem with your truck, the scanner will give you a code. To know what the code means, you will have to go through your manual. When the test is over and you have the results, you can move on to repair or replacement.

Step 4: Do a Voltage System Check

do a voltage system check

The next step is to use a Multimeter to do a voltage system check to make sure the battery voltage is correct as well as all grounds. This includes the battery computer as well as body grounds.

Step 5: Check the Level and Condition of the Fluid

check the level and condition of the fluid

Check the level as well as the condition of the fluid to make sure that the level is correct and verify the condition of the fluid.

Pull the transmission dipstick out, wipe it off, and insert it back in the tube. Remove it again and check the dipstick. Make sure you check both sides of the dipstick. The lower side is your correct reading. If everything is okay externally, you should then feel pretty sure that that the problem is inside your transmission.

Diagnostic Transmission Codes

In some cases, you may replace the transmission when the problem is not in the transmission. The malfunctioning could be due to an external sensor, engine misfire, or even solenoid.

Before replacing the transmission it’s important to do an accurate diagnosis. Always connect a scanning tool to your truck’s OBD II port to look for diagnostic trouble codes. If the codes are there, do in-depth research to find out exactly what they mean.

Also, monitor the truck’s parameters to make sure that they are reading correctly. For example, MLPS or a manual level position sensor can cause erratic shifting.

Most Common Transmission Codes


The P0700 code is a sign that your transmission control system has malfunctioned or failed. Some of the common indications you will notice include transmission slipping, erratic shifting, hesitation during acceleration, no response when stepping on the gas, limp mode, check engine light, delayed shifting, irregular shifting, as well as poor gas mileage. 

Check transmission fluid levels, check the color of the fluid to make sure that it isn’t cloudy. Also, you should go ahead and check for transmission fluid leaks.


This is a diagnostic trouble code that indicates a fault in communication between the TCM and the Engine Control Module (ECM). In other words, there’s a compatibility issue between your truck’s TCM and ECM. If this happens, you need to repair the truck immediately.


This is a diagnostic trouble code for transmission over temperature. It means that the transmission temperature has risen beyond the maximum limit as directed by the manufacturer. 

This code could be triggered by internal damage, a leak in the transmission, or a faulty transmission temperature sensor.


This is a diagnostic trouble code that is triggered when there is a malfunction in the TCM processor. This code means that the PCM of your truck has detected that your TCM system isn’t working properly. 

You could have a defective TCM, shortened or open TCM harness, and corroded or faulty connectors.


This is a diagnostic trouble code that indicates the transmission range sensor circuit isn’t sending the ideal data to the ECM. The code could be triggered by old/dirty transmission fluid, damaged/defective valve body, defective/damaged park/neutral position, damaged/faulty linkage, defective/damaged transmission sensor among other things.


This is a diagnostic trouble code for the malfunction of the shift solenoid. This code indicates that the PCM has picked up a fault with the shift solenoid. This code is usually caused by faulty valve body assembly, low transmission fluid level, faulty shift solenoid among other things.


Can TCM Duramax Be Repaired?

A failing TCM can be repaired and it may cost approximately $300, which is slightly lower compared to the cost of replacement. But a damaged Transmission Control Module has to be replaced to avoid incurring higher repair costs.

Can I Replace My TCM at Home?

It is always advisable to get the services of an experienced mechanic, which should cost you approximately $50 to $200. The aftermarket TCM should cost roughly $500 to $900, while parts may cost about $450 to $700.

Can a Truck Run Without a TCM?

If you drive a truck without a Transmission Control Module, it would not be able to change gears. This could lead to mechanical issues as well as subpar driving experience.

See also: Basic Guide to Tuners

Final Thoughts

This guide should help you learn how to test TCM Duramax. But the test and diagnosis can be quite tricky if you don’t know what to look for.

To help make the process simpler, study the different types of diagnostic trouble codes that you should look for on the scanning tool. You also need to look out for common symptoms of a bad transmission.

Always test and diagnose your TCM Duramax before you consider a replacement.