While engine trouble codes are somewhat well understood these days, automatic transmission codes are still confusing many out there. One of the reasons for this is the lack of uniformity between different transmission systems, which often makes troubleshooting them somewhat tricky. Today we’ll talk about the P0740 transmission code. 

This is a check engine light code that could potentially make your vehicle difficult, or impossible to drive altogether. Today we’ll take a closer look at this error, what causes it, the most common symptoms you’ll be dealing with, and how to diagnose it. 

What Does the P0740 Code Mean? 

The P0740 code means that your transmission control module, the computer tasked with controlling your automatic transmission, has registered a problem with the torque converter clutch circuit. In other words, your torque converter is not functioning normally, and the cause is most likely a mechanical issue of some sort. 

One positive thing about this particular error is that its causes are usually benign. That being said, you could potentially run into this code when your torque converter has suffered a more serious mechanical failure, at which point you’ll probably have a few more errors logged in your ECU’s error feed. 

What Is a Torque Converter? 

Since most of the issues resulting in the P0740 code have something to do with your torque converter, we should do a quick rundown on what this component of your car’s transmission does. The torque converter is a crucial piece of any powertrain that uses an automatic transmission. It’s essentially a form of hydraulic linkage that replaces the mechanical clutch assembly found on manual cars. 

The torque converter works by utilizing transmission fluid to transfer the power from the engine to the transmission. It consists of a pump, a reactor, and a turbine. The pump and turbine look very similar as they both feature metal fins fanned all around their circumference. The main difference between these two components is the orientation of the fins and the components they are attached to. 

The pump is driven by the engine, while the turbine drives the transmission. The whole system works as follows — the engine spins the pump, which spools up the fluid inside the torque converter. The fluid is then passed through the reactor where its direction of flow is optimized before it hits the turbine. The turbine then sends the power further to the automatic gearbox. 

What Does the Torque Converter Clutch Do? 

Modern torque converters are fitted with a clutch assembly whose sole purpose is to lock the pump side and the turbine side, thus allowing them to move at the same speed. While a torque converter doesn’t need this clutch to operate, the presence of a clutch makes the whole assembly more efficient. 

The torque converter clutch solenoid, which we’ll be discussing today as one of the causes of the P0740 error, activates and deactivates the clutch as necessary. 

What are the Causes of the P0740 Code? 

There are several things that can happen with your transmission that will trigger a P0740 DTC. Fortunately, the less severe issues are by far the most common ones. Here’s a list of said causes sorted by their severity: 

• Lack of transmission fluid
• Worn or contaminated transmission fluid
• Saturated transmission fluid filter
• Torque converter clutch solenoid failure
• Damaged torque converter or clutch assembly
• A foreign object inside the transmission assembly 
• Internal transmission damage
• Bad TCM or PCM 

Although this list looks fairly long and complicated, it’s worth mentioning that about half of the issues on there are usually caused by improper transmission maintenance. When you don’t service your transmission on time, the old fluid loses much of its efficiency. Not only that but there’s a decent chance that your transmission fluid filter becomes clogged if you skip enough fluid changes. 

Any debris inside your ATF can cause issues with the TCC solenoid, as it too relies on good fluid to operate as designed. A stuck TCC can cause mechanical damage to the clutch assembly if it’s constantly engaged. 

Then again, if the clutch fails to activate, you could experience a transmission fluid over-temperature condition as the presence of a clutch serves to cool the AT fluid inside the torque converter. 

Symptoms of a P0740 Code 

Considering the wide range of potential causes that can trigger this error code, it’s only normal to expect a diverse set of symptoms. There is a silver lining here, though. Different symptoms can tell you with some certainty what is causing your problem. Here’s the list of the most common ones. Later, we’ll discuss what each could potentially mean: 

• The transmission has trouble shifting in or out of certain gears
• Rough shifting
• The car won’t move
• Engine stalling when you pull to a stop
• Overheating transmission
• Terrible gas mileage 

Here’s what some of these symptoms could be telling you. If your car refuses to move, or it keeps stalling when you pull to a stop, there’s a chance your clutch assembly is stuck in an engaged or disengaged position. This would further implicate a bad torque converter clutch solenoid. 

If your transmission is overheating, that usually means that the clutch is not activating when necessary, further implicating the TCC solenoid. 

If your car is having trouble shifting gears, or it has trouble staying in gear, there’s a chance your transmission fluid is worn, or low. 

How to Fix the P0740 Error? 

Fixing this error should always start with a full diagnostic scan. You’ll need an OBD-II scanner for this. Plug the device into your car and run a full scan. 

For once, you’ll need to do this to even confirm that you’re dealing with a P0740 in the first place. More importantly, the presence of any other errors can point you in a diagnostic direction that you may not have thought of on your own. For example, if the P0740 appears along with a U0101 error, there’s a decent chance that your TCM or its wiring harness is what’s causing the issue. 

Inspect the Fluid 

Once you’ve run a full scan and got the errors off your OBD-II scanner, the next thing to do is check the AT fluid. You’re looking for a bright red color and proper fluid levels. Dark red color, burnt smell, and low fluid levels could indicate a leak somewhere, or lack of maintenance if you’re doing all of this as a part of a pre-purchase inspection. 

Inspect the TCC Solenoid 

Assuming that the AT fluid is in a good condition, proceed to inspect the TCC solenoid and its corresponding connector/wiring. Most of the time, you’ll find that this solenoid is faulty and in need of replacement. 

Fortunately, these solenoids are very easy to test given that you can reach and remove them from the transmission. Consult your car’s service manual to find out where your TCC solenoid is located. 

With the TCC solenoid dismounted and sitting on your test bench, take your multimeter, select the resistance setting (Ohm symbol), and measure the resistance on the solenoid. If you’re getting back zero Ohms, or if you’re getting a crazy high number of Ohms, your solenoid is probably bad. You’re looking for values anywhere between 12-28 Ohms depending on the make and model of the solenoid. 

You can also test if the TCC solenoid is functioning by hooking it up to a 12V power source. Remember to be extremely careful when doing this as shorting the leads can fry the solenoid, and then you’ll definitely need a new one. If you can hear and see the solenoid clicking open when powered, it means you’re looking at a functioning unit. 

TCM or Torque Converter Issues? 

If the TCC solenoid is good and your fluid/fluid levels are good, there’s a chance that your TCM or torque converter has gone bad. Neither of these issues is easy to diagnose without taking the transmission apart. Errors in the CAN bus network could indicate an issue with the TCM, even if they’re a history or intermittent errors. 

The process of diagnosing the torque converter will differ from car to car. Consult your service manual for more information on this subject. 

Can you drive with the P0740 code?

If you’re lucky and your clutch isn’t stuck, you could potentially drive the car with the P0740 present. However, that doesn’t mean that you should. On the contrary, only do so if it means getting the car to a shop/home in absence of other options (tow truck, help from a friend, .etc)

Keep in mind that this error can indicate serious issues with your transmission. Even if you can drive the car, there’s no saying how much damage you’ll be doing in the process. Because of that, it’s best to address the issue before continuing to use that particular vehicle.

Use Only Quality Parts to Service Your Transmission

The best way to avoid dealing with the P0740 error is to prevent it from happening in the first place. You can do this by servicing your car’s transmission on time using quality fluids/parts. Here at eEuroparts.com, we carry one of the largest catalogs of transmission parts, fluids, and other related products aimed at European vehicles. 

We also stock TCC solenoids for a number of European makes and models. Head over to our store, select your vehicle using our car selection tool, and you’ll be able to find parts that are a guaranteed fit for your car!