What P0840 means
Transmission fluid pressure sensor/switch A and B are inputs inside the automatic transmission.
How the engine computer detects it
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.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- 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
- Pressure switch or sensor A faulty or stuck Most likely
- Sticking shift valve or dirt in the valve body Possible
- Harness or connector at the transmission: fluid wicking, corrosion, chafing Possible
- Fluid level low or fluid contaminated Possible
- Transmission control module faulty Less likely
How to diagnose P0840
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: transmission fluid pressure sensor, transmission valve body, transmission wiring, transmission control module.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 8
- Transmission
- 40
- Specific fault
Questions drivers ask
Can I drive with code P0840?
If the car shifts normally, drive gently to a shop. If it's stuck in one gear, slips, shifts harshly or won't move out of Park, don't drive it: have it towed.
Will my car pass an emissions inspection with P0840?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.