Meaning, Causes, Diagnosis, and Solutions for P0840 Error Code

The P0840 error code is a diagnostic trouble code that indicates a problem with the transmission fluid pressure switch/sensor “A” circuit. This code is triggered when the Engine Control Module (ECM) detects a low or abnormal reading from the transmission fluid pressure switch. In this article, we will discuss the meaning of P0840, its symptoms, causes, diagnosis mistakes, and fixes.

What Does P0840 Mean?

The P0840 error code indicates that the ECM has detected a low or abnormal reading from the transmission fluid pressure switch. This switch is responsible for monitoring the pressure of the transmission fluid in the crankcase. The fluid flows through various channels and chambers to enable smooth gear shifting.

Symptoms of Code P0840

The symptoms of a P0840 error code may vary depending on the severity of the issue. In some cases, the only symptom may be an illuminated engine warning light on the dashboard. However, other symptoms may 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.

Causes of Code P0840

There are several possible causes of a P0840 error code. The most common cause is a defective electronic pressure control solenoid. However, other possible causes include:

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

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How Serious is Code P0840?

The P0840 error code indicates a fairly severe issue. While some vehicles may not exhibit any symptoms other than an illuminated engine warning light, this code may cause various transmission issues that require immediate attention. If left unaddressed, the operation of the vehicle will decline and cause a series of drivability issues until it no longer moves.

Common Diagnosis Mistakes for Code P0840

One common diagnosis mistake when diagnosing a P0840 error code is replacing or repairing the high-pressure pump without checking the solenoids, switch, and electrical wiring harness and connectors. These components are easy to diagnose and should be checked first before replacing any major parts.

Tools Needed to Diagnose Code P0840

Diagnostic scanner
DVOM (digital volt/ohmmeter)

How to Diagnose Code P0840

When diagnosing a P0840 error code, the first step is to pull the code from the onboard computer and erase it. Then, run the vehicle and scan it again. If the scan tool pulls the code a second time, it indicates an underlying problem.

All transmission-related problems diagnosis should start with a transmission fluid check. Replace the transmission fluid if it is low. If it is dirty, burnt, or filled with debris, it indicates that the transmission needs a system flush. 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. Check other potential causes until the issue is rectified.

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