Fault code P0840 – transmission fluid pressure sensor/switch “A” circuit malfunction

Fault code P0840 is called “Transmission Fluid Pressure Sensor/Switch “A” Circuit Malfunction” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P0840

This diagnostic trouble code (DTC) is a generic transmission code. Error P0840 is considered a general code because it applies to all makes and models of vehicles. Although the specific repair steps may vary slightly from model to model.

Fault code P0840 – transmission fluid pressure sensor/switch "A" circuit malfunction

The transmission fluid pressure sensor (TFPS) is usually attached to the valve body, inside the transmission. But in some vehicles, it can be found screwed into the crankcase or transmission case.

The TFPS converts the transmission’s mechanical pressure into an electrical signal for transmission to the transmission control module (PCM). Typically, the PCM / TCM then informs the other controllers using the vehicle’s data bus.

The PCM / TCM receives a voltage signal to determine the transmission operating pressure or when a gear shift occurs. This code is set if input “A” does not match the normal operating voltages stored in the PCM / TCM memory.

Sometimes the problem can be related to internal mechanical problems within the transmission. But more often, error P0840 is a problem with the electrical circuit of the TFPS sensor. This should not be overlooked, especially when dealing with an intermittent problem.

Troubleshooting actions can vary depending on the manufacturer, type of TFPS sensor, and wire colors.

Symptoms of vehicle malfunctions

The main signal that an error P0840 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. Check engine control lamp on the control panel will light up (the code will be recorded in the ECM memory as a malfunction).
  2. The vehicle starts in 2nd or 3rd gear (emergency mode).
  3. Unable to shift gears.
  4. Hard gear shifting or transmission slipping.
  5. Transmission overheating.
  6. Problems with engagement and disengagement of torque converter lockup clutch.
  7. Increased fuel consumption.

The severity level of error P0840 is quite high, so it is recommended to eliminate the problem as soon as possible. Because ignoring the problem for a long time can lead to more extensive and costly repairs. Over time, the performance of the vehicle can decrease so much that it simply cannot be driven.

Factors that can cause this error code

The error code P0840 can mean that one or more of the following problems have occurred:

  • There may be a short to ground fault in the TFPS sensor signal circuit.
  • Probability of TFPS sensor malfunction (internal short circuit).
  • Contamination or low level of ATF transmission fluid.
  • Clogged or clogged transmission fluid passages.
  • Mechanical failure inside the transmission.
  • Sometimes the cause is a faulty PCM module.

How to fix or reset OBD-2 code P0840

Some suggested steps for troubleshooting and fix the error code P0840:

  1. Change ATF fluid and filter.
  2. Fill transmission fluid to required level.
  3. Flushing to clean internal ducts of the transmission.
  4. Testing, if necessary replacing faulty pressure control solenoid valve.
  5. Troubleshooting mechanical problems inside the transmission.
  6. Checking connectors as well as wiring.
  7. Flashing or replacing the PCM.

Diagnose and repair of problems

Always check the technical service bulletins (TSB) for your particular vehicle to get started. The P0840 problem may already be a known problem with a known fix issued by the manufacturer. This can save you time and money during diagnosis.

The next step is to locate the transmission fluid pressure sensor (TFPS). Once found, visually inspect the connector and wiring. Look for scratches, scuffs, exposed wires, burn spots, or melted plastic.

Disconnect the connector and carefully inspect the terminals inside the connector. Look to see if they look burnt or have a green tint indicating corrosion. If you need to clean the terminals, use an electrical contact cleaner and a plastic-bristle brush. Allow to dry and apply electrical grease to the contact areas of the terminals.

Using a diagnostic tool, clear the trouble codes from the memory and see if the P0840 error returns. If the code returns, we will need to test the TFPS sensor and associated circuits.

Wiring test

With the ignition key off, disconnect the electrical connector on the TFPS sensor. Connect a digital voltmeter. The black wire to ground and the red wire to the signal terminal on the TFPS sensor wiring harness connector.

Turn on the ignition key. Depending on the manufacturer’s specifications, the voltmeter should show either 12 volts or 5 volts. Jerk the connection and see if the reading changes. If the voltage is incorrect, replace the faulty wires or connector.

Sensor Test

Connect one ohmmeter lead to the signal terminal on the TFPS sensor, and the other to ground. Check the resistance against the manufacturer’s specifications. Jiggle the connector on the transmission fluid pressure sensor, checking the resistance. If the ohmmeter readings differ from the manufacturer’s recommendation, replace the TFPS sensor.

If the wiring and sensor tests are done, but you keep getting a P0840 code. You need to look at the PCM / TCM module as well as internal transmission failures. But you need to do this after replacing the TFPS sensor. More often than not, it is the one that is the problem.

If you are unsure, ask a qualified automotive diagnostician for help.

On which vehicles does this problem occur most frequently

Fault code P0840 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • Chevrolet
  • Chrysler
  • Dodge
  • Honda
  • Mitsubishi
  • Nissan (Murano, Qashqai, X-Trail)
  • Toyota
  • Volkswagen (Touareg)

Fault code P0840 can sometimes be found with other errors. The most common are the following: P0335, P0780, P0841, P0842, P0843, P0844, P0845.


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