P0743 Code (Symptoms, Causes, and How to Fix)
With a P0743 code, it may seem like your car has forgotten how to properly shift gears or even stall. It’s one of several transmission-related OBD-II trouble codes that can affect automatic transmission vehicles.
Let’s go over some of the most likely causes of a P0743 code along with recommended step-by-step diagnostics to identify the actual culprit and fix the problem.
What Does Code P0743 Mean?
Diagnostic fault code P0743 describes a fault pertaining to one or more issues surrounding the manner in which a vehicle’s torque converter is “locking up”, in response to electrical signals present at the torque converter lockup solenoid.
More accurately, the torque converter lockup does not appear to be occurring as anticipated, despite the electrical command to do so. To fully understand this principle, you must first familiarize yourself with the operation of an automatic transmission’s torque converter.
A torque converter functions as the coupling between a vehicle’s engine and transmission. This coupling is viscous rather than mechanical under normal operating conditions, allowing controlled slippage while sitting at idle with the brake applied.
To prevent excess power loss and heat build-up at higher speeds, torque converter lockup takes place. When this occurs, a mechanical, rather than viscous coupling between engine and transmission is created. This is achieved when the converter clutch is engaged, via actuation of the lockup clutch solenoid. This solenoid itself is generally supplied with 12v power and ground via the vehicle’s PCM.
In the case of diagnostic fault code P0743, a lack of torque converter lockup is being detected by the affected vehicle’s PCM, despite the delivery of a signal to the torque converter lockup solenoid to facilitate such action.
In turn, torque converter lockup is unable to be facilitated in a meaningful manner thereby leading to a host of driveability-related concerns.
Related Codes: DTC P0740, DTC P0741
Symptoms of Code P0743
Diagnostic fault code P0743 is often accompanied by at least one secondary symptom, if not more. While some of these symptoms can be quite concerning, they can also point motorists in the direction of the issue at hand.
- Illuminated check engine light
- Stalling at stops
- Reduced fuel economy
- Failure to shift into highest gears at speed
- Shuddering at startup/shutdown
Causes of Code P0743
Diagnostic fault code P0743 can be caused by a host of underlying issues, some of which tend to be simpler than others to address. However, those armed with knowledge surrounding these potential causes often prove to be a step ahead in the diagnostic process.
The following are several of the most likely causes of DTC P0743.
- Faulty torque converter lockup solenoid
- Failed torque converter
- Defective engine coolant temperature sensor
- Issues within the transmission valve body
- Low, dirty, or improper transmission fluid
- Compromised PCM
Is Code P0743 Serious?
Generally speaking, most consider diagnostic fault code P0743 to be of at least moderate severity. This stems directly from the fact that this fault can prove quite detrimental in terms of vehicle driveability. It is also highly unlikely that the symptoms of this fault will improve as time passes.
In the most severe of cases, DTC P0743 has been associated with vehicle stalling when approaching a stop. While this is concerning in itself, some motorists have also experienced issues pertaining to an inability of the affected vehicle to shift into high gear at road speeds. Therefore, additional stress is placed upon the engine during high-speed operation.
Because of the potential for stalling and inability to shift properly, it’s not recommended to continue driving until the issue is resolved.
How to Fix Code P0743
The steps below can be followed to assist in diagnosing and repairing the root cause of your vehicle’s P0743 fault code. It is also considered best practice to have a factory-specific service manual for your particular model of vehicle at the ready, before attempting any such repairs.
#1 – Check For Additional Trouble Codes
Before delving into the diagnostic process itself, check for the presence of any additional diagnostic fault codes. Any such codes that are present should be recorded and diagnosed thoroughly before proceeding.
#2 – Inspect Transmission Fluid
Begin the diagnostic process by carefully checking your vehicle’s transmission fluid level. Make sure that the fluid is filled to manufacturer specifications and that it doesn’t appear dingy, contaminated, or burnt.
If the affected vehicle’s transmission fluid appears questionable in quality, a fluid and filter change is recommended.
#3 – Replicate Concern
You will now need to drive the vehicle in question, in an attempt to replicate the issue at hand, so that all pertinent freeze frame data can be reviewed.
In order to best replicate this issue, try driving at various speeds, across mixed grades. While driving, pay close attention to any anomalies that become evident.
#4 – Analyze Freeze Frame Data
At this point, freeze frame data from your drive can be reviewed via a good scan tool (not all OBD2 scanners allow this). You should compare critical data logged in the moments leading up to the activation of DTC P0743, to ranges specified by the vehicle’s manufacturer.
Some of the most critical points of interest during this comparison will be engine coolant temperature, transmission temperature, and transmission slip rate.
#5 –Follow Manufacturer Directives
At this point in the diagnostic process, continued troubleshooting will be highly dependent upon manufacturer recommendations. Each manufacturer specifies their own series of tests for qualifying/condemning transmission-related components and sub-assemblies.
Therefore, it’s extremely important to consult factory-specific service literature for your vehicle before continuing. Doing so will help prevent costly misdiagnosis as this info is not a one-size-fits-all approach.
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