Error Code P0743 is defined as Torque Converter Clutch (TCC) Solenoid Circuit Electrical. This means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) is seeing a fault in the TCC, which is contained inside the transmission.

This code is a generic trouble code, meaning it applies to all vehicles equipped with OBD-II system, or vehicles made since 1996 up to present. Specifications on the definition, troubleshooting and repairs of course, vary from one make and/or model to another.

Definition

The job of the TCC is to increase make 1 to 1 RPM ratio between the Transmission Input Shaft and the Torque Converter’s rotational speed, so the manual transmission-like “mechanical lock” between the transmission and the engine is established. This way, loss of power that may occur with the fluid and/or hydraulic lock that usually happens in the conventional Torque Converter is eliminated. Also, since the engine is running at a reduced speed, emission output and overall fuel consumption are both reduced.

This transfer of power however, results in  excessive heat. Lockup torque converter works by making the transmission more efficient at highway speed and control heat. The TCC is what lockups the converter. Heat is what kills transmission, which is why we usually hear that stop and go traffic is hard on the transmission.

In the case of Error Code P0743, the PCM is reading higher than 200RPM difference between the rotational speed Torque Converter and the Transmission Input Shaft. When the Torque Converter Lockup Clutch engages, the ratio between the converter RPM and Input shaft RPM must be 1:1.

Common Symptoms

As with other error codes, Error Code P0743 activates the Check Engine light, which is one of its most obvious and primary symptoms. Other symptoms include:

  • Shudder upon take off or when coming to a stop
  • Engine stalls when coming to a stop after cruising speeds
  • Vehicle doesn’t shift into highest gear at freeway speed
  • Decrease in fuel economy

In some cases, the vehicle may not exhibit unusual or adverse conditions. While in other cases, some vehicles may show drivability problems, such as misfire-like symptoms or engine dying when coming to a stop.

Possible Causes

There are many factors that may lead to the occurrence of Error Code P0743, which include:

  • Dirty or low transmission fluid
  • Failed TCC solenoid
  • Failed TCC assembly in torque converter
  • Short or ground problem in circuits or wires
  • Defective PCM (rare)

How to Check

As with other diagnostic codes, diagnosing this error code requires recording of freeze frame data, and then duplicate the conditions (engine load, speed, throttle position, and RPM) when the code was set through a test drive to find out if the code comes back.

Conduct an overall fluid condition and check if the levels are okay. Also, check whether the transmission shifts correctly, and if the engine is running properly. In most cases, transmission problems are misdiagnosed due to poor engine running condition. Remember the flow starts at the engine and goes through the transmission then out to the drivetrain.

Then, check the external connectors and transmission harness. Also, check your fuses and make sure transmission case is grounded to both the body and the battery.

Then, find the right wires for the torque converter clutch. In many cases, it will be two wires; one for ground and one for power. Apply power and ground and notice the sound of the solenoid click.

If you didn’t hear a click, proceed on checking the continuity across the solenoid, to and from the corresponding wires to the sensor. The solenoid should have a low ohm (0.02 to 0.05) reading. Obviously, this varies from one manufacturer to the next, so consult with your factory service manual. The wires should also have a low ohm reading. Make sure both wires are NOT shorted to ground or power.

If the solenoid clicks, then the problem must be in the PCM. However, you should still examine the TCC solenoid for debris. Any small debris in it can cause is to short and trick to computer into thinking it is applied. Or, it can attach itself to the solenoid and keep it from disengaging. This is usually the case.

Also, it helps to get the latest calibration for your vehicle’s PCM from your dealer. Vehicle manufacturers constantly release new version of the software, and there may be a revised calibration to address the code, especially if the problem is intermittent.

If everything seems fine, that means you may need to replace your PCM. Don’t forget that it needs to be programmed to the vehicle. And in many, it comes with an antitheft device in the key, which must be programmed as well.

If all else fails, then your problem in the torque converter must be in the clutch assembly. This may be evident if you slip when the transmission goes into lockup in overdrive. Also, your fluid may smell burnt or look totally black. Check for a replacement or a rebuilt transmission.

How to Fix

Obviously, repairs for this error code depend on its diagnosis. Common fixes for this error code includes:

  • Total rebuild, fluid flush or replacement of the torque converter
  • Shorted or burned wires must be fixed or replaced
  • If there are discrepancies, replace full internal transmission harness
  • Fill the transmission with the right fluid
  • Replace pump
  • Repair or replace system circuitry, connectors, and/or components that doesn’t coincide with specs set by the manufacturer
  • Repair or replace system circuits/connectors as required and retest the system to ensure that repairs were successful
  • Replace PCM

Before you start addressing transmission problems, make sure the engine is in good running shape. If there are other sensor codes present, or lean/rich codes, they must be fixed first.

In many cases, a simple fluid and filter change can fix the problem.