P0840 code meaning
Transmission Fluid Pressure Sensor/Switch A Circuit
Powertrain system Generic code P08xx codes Check engine light
What P0840 means
Transmission fluid pressure sensor/switch A and B are inputs inside the automatic gearbox. A switch only reports pressure present or absent, a sensor reports a value. The letter is the maker's choice: Allison 1000 uses A and B for the switches on shift valves S1 and S2, GM's CVT for the primary and secondary pulley sensors, GM's 2025 8-speed A for the lube pressure sensor; Ford reports pressure faults under its solenoid codes instead. P0840 is the general circuit fault. GM documents it for the 2006 Allison: the module compares the commanded position of valve S1 with the switch feedback and retries the valve if they disagree; it then tells an electrical, mechanical or sporadic case apart. Type A, MIL after one trip. From 2007 Allison reports the same test under P0842/P0843.
P0840 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyHard
- Typical labor1.5 to 5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P0840
- Check engine or transmission warning lamp on
- Limp mode with a fixed gear, harsh or missing shifts
- Torque converter clutch stays open, higher fuel use
Common causes of P0840
- Most likelyPressure switch or sensor A faulty or stuck
- PossibleSticking shift valve or dirt in the valve body
- PossibleHarness or connector at the gearbox: fluid wicking, corrosion, chafing
- PossibleFluid level low or fluid contaminated
- Less likelyTransmission control module faulty
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Transmission fluid pressure sensorTransmission valve bodyTransmission wiringTransmission control module
What each part of P0840 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 8 | Area: transmission. |
| 40 | The exact fault in that area. |
What to do next
- Note P0840 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0840
What does P0840 mean?
P0840 means "Transmission Fluid Pressure Sensor/Switch A Circuit". Transmission fluid pressure sensor/switch A and B are inputs inside the automatic gearbox. It is a generic code, so it means the same on every make.
Is it safe to drive with P0840?
Drive gently and only to a workshop. With P0840 the car can go into limp mode (reduced power) to protect itself.
Will P0840 fail an emissions test?
Yes, it can. P0840 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0840 to fix?
Difficulty: hard. A typical repair takes 1.5 to 5 hours of work. This is a job for a workshop.
Will P0840 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.