P0840 Error Code – Meaning Causes Diagnoses and Fixes

On most modern cars, coupes, SUVs, trucks, the transmission and its internal components are regulated and monitored by various switches and sensors. To keep you ever alert, they signal the powertrain control module [PCM] every millisecond.

One of these regulating switches is the transmission oil pressure switch, which regulates and monitors the pressure of the transmission fluid flow in the crankcase.  This fluid flows through various channels and chambers to enable smooth gear shifting.

Like any other switch, it can wear off or fail at any time. Suppose the transmission fluid pressure switch fails, the onboard computer will set a series of error codes. One of the codes is the P0840 error code.

This article will explain the meaning of P0840, symptoms, causes, diagnosis mistakes, and fixes. Sit back and have a 5 minutes read.

Code P0840 Definition

Transmission fluid pressure switch/sensor “A” circuit

p0840 code

What Does P0840 Mean?

Diagnostics trouble code [DTC] P0840 means the ECM detects a low or abnormal reading from the transmission fluid pressure switch.

A transmission works with hydraulic pressure, regulated by solenoids, controlled by the car computer to offer excellent gear shifting and provide torque converter lookup. The ECM uses vehicle speed and other factors to determine the required pressure for overall vehicle optimization.

A transmission fluid pressure switch/sensor [TFPS] converts the mechanical transmission fluid pressure into an electrical signal for the transmission control module [TCM] or powertrain control module [PCM] to relate with the readings. The TCM/PCM will signal other system components that work with this signal.

Once the voltage signal gets to the TCM/PCM, it determines when a shift is happening or the operating pressure. The car computer triggers the P0840 fault if the “A” input does not correspond with the voltage signal stored on the PCM. The onboard computer can also trigger it because of internal transmission mechanical issues. Consult your vehicle service manual to determine your vehicle “A” circuit.

What Are the Symptoms of Code P0840?

On some vehicles, you may not notice any symptoms rather than an engine warning light illuminating on the instrument panel. However, the signs of a P0840 fault code include;

  • Bad gas mileage
  • Overheated transmission
  • Loss of shifting ability
  • Slipping transmission or hard shifting
  • Engagement or disengagement issues from the torque converter clutch
  • Gear ratio, torque converter, transmission lock up, slippage codes being registered on the car computer.

What Cause of Code P0840?

The onboard diagnostic computer can set the error code P0840 for several reasons. The most common cause is defective electronic pressure control solenoid. In any case, dirty, and low transmission fluid are common causes of P0840 on Nissan Altima. And other general causes include;

  • Mechanical internal transmission failures
  • Clogged transmission fluid galleries
  • Lousy TCM or PCM (in rare cases).
  • Faulty transmission pump
  • Failing pressure regulator
  • Defective transmission fluid pressure switch
  • Damaged or frayed wiring harness and connectors
  • Valve body issues
  • Failing pressure control solenoid.

How Serious Is Code P0840?

While P0840 fault codes may not show any symptoms on some cars, this code still shows a fairly severe issue. Be warned, it may cause various transmission issues needing urgent attention.

Vehicles with P0840 may run fine, but as time goes on, the operation will decline and cause a series of drivability issues until it doesn’t move any longer. Always fix this problem whenever you notice it, to avoid getting stranded in the middle of nowhere.

Code P0840 Common Diagnosis Mistakes

The mechanics make a common diagnosis mistake when diagnosing error code P0840. That is replacing or repairing the high-pressure pump without checking the solenoids, switch, and electrical wiring harness and connectors. The solenoids, switches, and electrical harnesses, and connectors are easy to diagnose and should be checked first before heading to any significant parts.

Tools Needed to Diagnose Code P0840

  • Diagnostic scanner
  • DVOM [digital volt/ohmmeter]

How to Diagnose Code P0840

When diagnosing a P0840 Nissan Rogue or any vehicle model, you need to pull out the code from your onboard computer and erase it. After that, the test runs the vehicle and scans it again. If the scan tool pulls the code the second time, it refers to an underlying problem.

All transmission-related problems diagnosis should start with the transmission fluid check. Replace it if you find the transmission fluid is low. And if it is dirty, burnt, or filled with debris, it shows the transmission needs a system flush. And you need to refill it with new fluid to the recommended level. 

After carrying out this or other repairs, erase the error code from the PCM and re-scan the onboard computer again. This process is to help you track down the cause of the problem. If correcting the transmission fluid issues did not fix the problem, it’s time to check the electrical wiring harness and connectors. Do not forget to check other potential causes until the issue is rectified.

How do you fix P0840?

Whether you notice P0840 on Honda CRV or P0840 Nissan, here are the possible ways to fix it;

  • Refilling, or flushing and replacing old transmission fluid
  • Unclogging blocked transmission fluid galleries
  • Replace transmission filter
  • Fix transmission fluid leaks
  • Repair or replace lousy wiring harness or connectors
  • Replace pressure regulator
  • Replace pressure control solenoid
  • Replace transmission pump
  • Replace or repair valve body
  • Replace defective torque converter
  • Replace transmission fluid pressure sensor
  • Replace or reprogram faulty TCM or PCM.

Same Problems with different Error Code

Here are error codes that show the same or related problems with the P0840 fault code;

  • P0841
  • P0842
  • P0843
  • P0845
  • P0846
  • P0847
  • P0848
  • P0849
  • P084A
  • P084B
  • P084C
  • P084D

Last words

The cause of P0840 could be a simple or a more complex issue. The first thing to do when you notice the symptoms above is to read the onboard diagnostics computer and address the code. We’ve also outlined the potential causes above and how to fix them.

You can carry out the repairs yourself, but if you’re not confident of getting your hands dirty, consult ASE-certified mechanics to diagnose and fix the problem.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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