P0743 is a generic OBD2 code that can occur in the Toyota Mirai. It indicates that your ECU/TCM has found an issue with the torque converter clutch solenoid.
It is a generic code (meaning it has the same definition for all cars and trucks). Repair steps are going to vary depending on your Mirai’s model year and transmission type.
Definition: Torque Converter Clutch Circuit
Here’s the definition of P0743, broken down into its two parts:
Torque Converter Clutch
Your Mirai uses a clutch in the torque converter to lock the engine and transmission together at highway speed. When they are locked together, it allows for increased fuel economy, and the engine will run a few hundred RPMs lower than it would without it.
Modern vehicles have been using locking torque converters for over 30 years now.
The torque converter clutch is controlled by the ECM/TCM and turns off when you lower the vehicle speed beyond a certain threshold, push down the brake pedal, or if the vehicle downshifts.
When your Toyota Mirai’s ECM/PCM detects an issue with the torque converter clutch circuit, P0743 will be stored in its internal memory.
P0743 does not indicate if the torque converter circuit voltage is too high or too low, just that the circuit signal is outside of the normal operating range.
This broad definition means that there can be many potential causes (covered below).
Toyota Mirai P0743 Symptoms
Since P0743 is a transmission-related code, it won’t affect how the engine runs. But, it can really impact the way the transmission feels when you are driving.
If the torque converter is locking when it shouldn’t be, it can make driving your vehicle nearly impossible. Here are the most common symptoms:
- Slightly higher RPM at highway speeds (converter not locking)
- Stalling when coming to a stop (converter didn’t unlock)
- Overheating transmission (either scenario)
- Decreased MPG
- Won’t move at all (this doesn’t usually happen)
Toyota Mirai P0743 Causes + Diagnosis
Here are the most common causes of P0743. They are listed with an emphasis on what you can check at home.
Check the Transmission Fluid (Level and Consistency)
Start by checking your Mirai’s transmission fluid. Low transmission fluid can cause a drop in line pressure and can cause the torque converter locking solenoid not to function correctly.
The transmission fluid needs to be full AND CLEAN. If the fluid is black or shows signs that it is burnt, changing it might be enough to clear P0776.
A clogged transmission filter can cause pressure inside the transmission to drop too.
Automatic transmission fluid has a very tough job. It has to cool, lubricate, and transfer energy. It can go a long time without being changed, but if it overheats, that can be enough for it to cause P0743 in your vehicle.
Check The Wiring Harness for Damage
Check the wiring harness going to your Mirai’s torque converter lockup solenoid (if you can access it). Ensure that the harness hasn’t been damaged by debris from the road. If it’s damaged, repair it, clear the code, and see if P0743 returns.
Test the Lockup Control Solenoid
The lockup control solenoid is responsible for activating and deactivating the torque converter clutch. You can test it with a good multimeter. The video below will show you exactly how to go about doing so.
Here are some other issues that are known to cause P0743 in the Toyota Mirai:
- Bad TCM/PCM
- Bad Torque Converter
- Internal Transmission Failure
P0743 is most likely caused by a wiring issue with the torque converter clutch solenoid, wiring problem, or transmission fluid issues. Good luck fixing your Mirai!