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P0840 OBD-II Trouble Code: Transmission Fluid Pressure Sensor/Switch A Circuit

Check Engine Light

P0840 code definition

The P0840 code indicates that there has been an abnormal voltage reading from the transmission fluid pressure sensor circuit.

What the P0840 code means

In order to shift gears and provide torque converter lockup, the transmission uses hydraulic pressure, controlled by solenoids, which are controlled by the PCM. Using the engine speed and various other factors, the PCM calculates the necessary amount of pressure at any given time to optimize vehicle operation. If the desired amount of pressure, and the actual pressure reading, do not match, then the code P0840 will be stored.

What causes the P0840 code?

The most common cause of the P0840 code is a faulty electronic pressure control solenoid. However, it is also common for dirty, contaminated, or low transmission fluid to be the problem. Other causes can include:

  • Blocked transmission fluid passages.
  • Mechanic internal transmission failures.
  • A faulty PCM or TCM, in rare cases.

What are the symptoms of the P0840 code?

Some vehicle owners may notice no symptoms at all, and only observe the Check Engine lamp illuminating. However, the symptoms of the P0840 code can include:

  • Loss of shifting ability.
  • Lack of engagement or disengagement from the torque converter clutch.
  • Harsh shifting or a slipping transmission.
  • Overheated transmission.
  • Decreased fuel efficiency.
  • Other shift solenoid, transmission slippage, lock up, gear ratio, or torque converter clutch codes being stored in the PCM.

How does a mechanic diagnose the P0840 code?

After verifying the code with an OBD-II scanner, all transmission-related faults should begin with a check of the transmission fluid. If the fluid is too low, it should be refilled; if the fluid is dirty, filled with debris, or smells burnt, it may be that the entire system must be flushed, and then refilled with clean transmission fluid.

After this, or any other repair, is performed, it’s important to clear the code from the PCM, and retest the system. This process helps the mechanic narrow down the exact cause of the issue. If the transmission fluid fix did not cause the code to disappear from the PCM, then next step is to inspect the electrical components, and to continue working through possible repairs until the correct issue is found.

Common mistakes when diagnosing the P0840 code

Most of the time, the biggest error made in diagnosing and repairing the P0840 code is the tendency to replace the high pressure pump, which is never in fault as often as the sensor, solenoid, or the electrical components. It’s important to start with the small and more commonly at-fault items before replacing any major parts when diagnosing the P0840 code.

How serious is the P0840 code?

While some vehicle owners may not observe any symptoms, this code still indicates a serious issue that should be addressed right away. As time goes on, vehicle performance will decline until the vehicle is not drivable, and ignoring this trouble code could lead to greater issues down the road.

What repairs can fix the P0840 code?

Repairs to fix the P0840 code include:

  • Replacing the pressure control solenoid.
  • Refilling, or flushing and replacing, the old transmission fluid.
  • Clearing blocked transmission fluid passages.
  • Addressing any mechanic internal transmission failures.
  • Replacing a faulty PCM or TCM, in rare cases.

Need help with a P0840 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
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