Transmission Fluid Pressure Sensor/Switch A Circuit Malfunction
The transmission fluid pressure sensor circuit has a fault.
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What it means (plain English)
The sensor that measures the pressure of the transmission fluid is either not working properly or has a bad connection, affecting how the transmission shifts and operates.
What the computer is actually seeing
The powertrain control module (PCM) expects the transmission fluid pressure to be within a certain range during operation, typically between 30-80 psi (pounds per square inch). If the pressure measurement is out of this range or if there is a short/open circuit, the code sets.
What a healthy reading looks like
The normal transmission fluid pressure ranges from 30 to 80 psi under typical operating conditions; if the sensor reports below 10 psi or above 90 psi, it may trigger P0840.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 11. Check transmission fluid level and condition. If low, top off or service. Decision: If level is good, proceed to step 2. If low, correct and recheck code.
- 22. Inspect the wiring and connectors for the transmission fluid pressure sensor. Look for any visible damage or corrosion. Decision: If damaged, repair or replace the wiring. If no issues found, proceed to step 3.
- 33. Measure the voltage at the sensor connector with the ignition on (engine off). You should see 5 volts. Decision: If not, repair the wiring back to the PCM. If voltage is present, proceed to step 4.
- 44. Test the resistance of the transmission fluid pressure sensor. It should vary with fluid pressure, typically from around 1.0kΩ at low pressure to less than 300Ω at high pressure. Decision: If out of spec, replace the sensor. If within spec, proceed to step 5.
- 55. Check for any stored codes in the PCM. Look for other related DTCs such as P0868 or P0841. Decision: If other codes are present, diagnose them accordingly. If P0840 is the only code, the PCM itself may need further diagnosis or replacement.
Common causes
- Wiring harness issue (damaged or shorted)
- Faulty transmission fluid pressure sensor
- Poor connection at sensor or connector
- Low transmission fluid level or contamination
- Faulty powertrain control module (PCM)
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with code P0840?
It's best to avoid driving until this is fixed, as it can lead to transmission slipping and potential failure.
Will this code come back after repairs?
If the underlying issue is fixed, it shouldn't return.
Why does this code sometimes appear with other codes?
Transmission issues often have related problems, like low fluid or sensor failures that create multiple fault codes.
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