6 Symptoms Of A Bad Transmission Control Module (TCM)

The TCM or transmission control module controls all the sensors and solenoids in your transmission. Here's how to tell your TCM is bad and how much it costs to replace it.

Symptoms Of A Bad Transmission Control Module

A transmission control module or TCM is an essential electronic device presented in all modern vehicles with an automatic transmission.

The transmission control module gathers data from the engine and transmission to make the gear shifting in the transmission.

If your vehicle’s transmission control module fails, a variety of different problems can occur, so a properly functioning transmission control module is important for your vehicle. Let’s take a quick look at the signs to look for.

Symptoms Of A Bad Transmission Control Module (TCM)

The most common symptoms of a bad TCM are a check engine light or a transmission fault warning light on your dashboard. You may also notice shifting issues like unpredictable shifting or no shifting at all. Increased fuel economy is a common sign of a bad TCM also.

Here is a more detailed list of the signs of a bad or failing transmission control module to look for:

1. Check engine light

Check Engine Light

The most common symptom of a bad transmission control module is a check engine light on your dashboard.

The engine control unit and the transmission control unit work together. If the engine control unit receives a faulty signal from the transmission control unit – it will light up the check engine light.

2. Transmission fault light

Gearbox Symbol

Some cars do also have a transmission fault light on the dashboard. It is difficult for the transmission control module to know when it is damaged, but if another control unit like the engine control unit gets strange data from the TCM, it will light up the transmission fault light.

3. No Gear shifting at all

No Gear Shifting

Another common problem when it comes to a faulty transmission control module is no shifting at all. It will maybe stay in neutral or park without even trying to put in the first gear or reverse. If this happens, it is a sign of a faulty TCM.

You can also notice it if your car is shifting to first gear and going forward but refuse to shift up to the next gear.

4. Unpredictable shifting

Car Shifting Problems E1609782519191

If your TCM has gone bad, it can do a lot of strange things with the shiftings. It can downshift when it should not and upshift when it shouldn’t. This can cause dangerous situations in the traffic, so you should check it as fast as possible if you experience it.

5. Stalling engine on gear shifting

Broken Car

You can also notice that the faulty TCM is upshifting way too early, sometimes making your engine stall or almost stall.

This is also a dangerous situation on the road because you can lose control over your vehicle and you should fix it as soon as possible.

6. Poor fuel economy

Refuel Car

Poor fuel economy can also happen when the transmission control unit went bad.

The transmission control module is programmed to shift the gearbox for the best possible fuel economy. If this doesn’t happen, there is a risk that your fuel economy will suffer.

What is the function of the transmission control module?

Transmission Control Unit

The transmission control module, or TCM, is an electronic device similar to the ECU or engine control unit. However, the module’s functionality differs because the TCU is basically responsible for the proper operation of the transmission and transaxle.

The transmission control unit or module essentially receives information, in the form of sensor data, from several different sensors, which are then processed and manipulated to produce specific signals and commands.

These commands are transmitted to many other components to ensure the proper operation of the vehicle.

Transmission Control Module (TCM) Location

The transmission control module is usually located under the center console inside your car. It can also be located on the transmission itself.

This is very different depending on which car you have, as the transmission control module can particularly be located anywhere inside your car.

To find the transmission control module’s exact location for your particular car model, you should check the repair manual for your car model or ask your authorized dealer.

Transmission Control Module Inside Gearbox
Transmission control modules can sometimes even be installed inside of the gearbox.

Transmission Control Module Replacement Cost

The average transmission control module (TCM) replacement cost is between $250 and $1250, depending on the car model and labor costs. The average cost of the transmission control module is between $200 and $750 and the labor cost is between $50 and $500.

The costs associated with replacing the transmission control module in your vehicle depend on many factors, such as your vehicle’s design and type, the company, and the parts to be replaced.

In some vehicles, the transmission control module is even installed inside the transmission, which creates the problem that you have to take it apart. As you may understand, this can be very costly.

The other thing to consider is that after the transmission control module replacement, it needs to be programmed by a diagnostic tool. You most often need to do this at a workshop, and nothing you can do yourself with basic knowledge.

You can try to find a used TCM on the scrapyard to reduce the costs if you think the TCM is fried.

TCM Testing


Will a bad TCM throw a code?

Yes, but often the TCM is so bad that you won’t even be able to communicate with it with your scanner. If you can’t communicate with the TCM because it’s bad, you should be able to find a fault code in the engine control module that says it can’t communicate with the TCM.

What causes a TCM to fail?

There are many reasons why a transmission control module would fail. It could either be due to water and corrosion getting into the module, or it could just be due to age. A TCM is made to last the life of the car, but can fail due to a hardware or software issue.

Can a bad TCM be repaired?

If there is just a cracked solder or something similar in your TCM, it may be repairable. However, it depends on the severity of the damage to the TCM. To determine if your TCM is repairable, you will need to consult a professional to take a look at your control module.

Can you drive without a transmission control module?

No, you most likely won’t be able to drive your car at all without the TCM. Without the TCM, the automatic transmission will not engage any gear and your car will be stuck in Park or Neutral.

Conclusion: Bad Transmission Control Module Symptoms

The main symptoms of a bad transmission control module include:

  • Check engine light
  • Transmission fault light
  • No Gear shifting at all
  • Unpredictable shifting
  • Stalling engine on gear shifting
  • Poor fuel economy
Magnus Sellén
Written by:

Magnus is the owner and main author of Mechanicbase. He has been working as a car mechanic for over 10 years, and the majority of them specialized in advanced car diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.

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