P0840 Code: Transmission Fluid Pressure Sensor/Switch “A” Circuit

The transmission fluid pressure sensor relays information that the powertrain control module (PCM) or transmission control module (TCM) needs to engage the clutches and shift gears smoothly while driving. Fluid pressure in the system can be affected by problems with the hydraulic clutch or pressure switch.

If the PCM detects an abnormality with the transmission fluid pressure, on-board diagnostics (OBD) will log a P0840 code.

What Does the P0840 Code Mean?

Diagnostic trouble code (DTC) P0840 stands for “Transmission Fluid Pressure Sensor/Switch ‘A’ Circuit.”

The transmission fluid pressure sensor sends a voltage signal to the PCM, informing the computer of the hydraulic pressure within the transmission. Using this information, the PCM or TCM will control output devices, such as the pressure control solenoid.

The transmission fluid pressure sensor is usually subject to electrical fault, causing it to send operating voltage input that’s outside the normal range. This issue can trigger a P0840 code.

Note: The definition of code P0840 might be different depending on the vehicle manufacturer. Consult the appropriate repair manual or repair database for the exact code definition.

What are the Common Causes of the P0840 Code?

• Transmission fluid pressure sensor failure
• Wiring issues
• Low transmission fluid
• Powertrain control module failure
• Internal transmission failure
• Valve body problems

What are the Common Symptoms of the P0840 Code?

• Illuminated check engine light
• Transmission not shifting properly
• Increased fuel consumption
• Vehicle in limp mode

How to Diagnose the P0840 Code

Code P0840 is a generic powertrain code that vehicles with an OBD-II system can log. However, keep in mind that factors like vehicle specifications (year, make, and model) can alter the diagnostic process.

Diagnosing this type of code might involve the use of a scan tool and checking the affected system for damaged wiring. If you’re not familiar with how the process works, you can always let a certified mechanic do the job for you. But if you think you can handle the task on your own, then you can go ahead and perform the necessary procedures.

To help you out, here’s a short clip that shows what the process might involve.

How to Fix the P0840 Code

Resolving a P0840 code can be tricky, especially if you don’t have the right equipment and technical knowledge. If done incorrectly, your vehicle could develop more problems, which might lead to expensive repairs. If you’re not confident with your DIY skills yet, it’s best to bring your vehicle to the nearest auto repair shop and have a trained professional work on your daily driver.

But if you have some experience fixing problems like this one, then you already know that there’s not a single solution that works for all makes and models. So before you begin, make sure to consult the appropriate repair manual. Guides like those from Chilton or an ALLDATA subscription contain vehicle-specific repair information, so you might want to check those out.