P0740 is one of the most common transmission trouble codes. It means that the Serena‘s computer has detected that there is an issue with the torque converter clutch (TCC) solenoid circuit.
P0740: Torque Converter Clutch Circuit Malfunction
The TCC “locks” the drivetrain, which allows for a true mechanical connection between the transmission and the engine. In effect, this allows an automatic transmission to function like a manual transmission at certain speeds and engine loads, and eliminates all slipping from the transmission.
P0740 Symptoms: Nissan Serena
The symptoms of P0740 vary greatly. If the converter stays unlocked, it can be a nuisance. If it locks when it shouldn’t be, that’s where it can get dicey.
The most common symptoms include:
- Poor Gas Mileage- If the converter is not locking at speed, the engine’s RPM’s will stay higher. This decreases fuel mileage.
- Stutters and Stalls- This would be a symptom of the transmission locking when it should not be. It feels exactly like when you need to downshift in a manual transmission.
- Higher RPM on highway- If the converter is not locking, it can lead to higher RPM on the highway. This increases fuel mileage and makes the transmission operate at a higher temperature.
- Service Engine Soon Light
Here’s a great video on diagnosing this code:
P0740 Causes Nissan Serena
There are many different things that can cause the P0740 code. Most of them are going to need an experts touch to diagnose.
This list is presented with a combination of likely to cause the issue and repairability in mind first.
If there isn’t enough transmission fluid, the transmission will not be able to operate normally. There are many symptoms of low transmission fluid. The most noticeable one is going to be the transmission slipping.
If the transmission fluid is dirty or burnt, it can make the transmission behave in a way that is not normal and throw the P0740 code. Here’s a pretty good guide on transmission fluid color.
If the fluid is low or bad, replacing should be considered an urgent priority. The good news is that transmission fluid and filters are cheap.
The wiring harness that goes to and from the transmission is one of the more vulnerable pieces of wiring on a vehicle. Inspecting the harness for obvious signs of distress, checking the connection at the transmission itself, and finally a continuity test can go a long way in diagnosing your Serena.
It is highly likely that the torque converter clutch solenoid (TCC) has gone bad. But, jumping straight to this conclusion without inspecting the transmission fluid and wiring can cause a lot of undue headache.
The torque converter itself or torque converter clutch can go bad and throw the P0740 code.
Problem in the Case
If the TCC seems to be functioning properly, it may be that the problem is in the transmission case itself. It could be a valve body issue, a problem with the pressure going through the transmission, or a clogged filter.
Articles on this site are always written with the DIY enthusiast in mind. Most of the time, you’re not looking these codes up for casual reading. You’re looking them up to fix your vehicle. We would never want to discourage that. With that being said, P0740 is a tougher code to diagnose and is often out of the range of the shade tree mechanic.
If there is anything that you would like to add, please feel free to leave a comment.