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.
Medium severity. Can I keep driving?Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
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
Most likely causes
- Pressure switch or sensor A faulty or stuck
- Sticking shift valve or dirt in the valve body
- Harness or connector at the gearbox: fluid wicking, corrosion, chafing
- Fluid level low or fluid contaminated
- Transmission control module faulty
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: hard
- It can trigger limp mode.
- It affects emissions.
What each character of P0840 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 8: Transmission
- 40: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0840?
Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
What causes code P0840?
The most common causes are: pressure switch or sensor A faulty or stuck; sticking shift valve or dirt in the valve body; harness or connector at the gearbox: fluid wicking, corrosion, chafing.
Related codes
- P0841Transmission Fluid Pressure Sensor/Switch A Circuit Range/Performance
- P0842Transmission Fluid Pressure Sensor/Switch A Circuit Low
- P0843Transmission Fluid Pressure Sensor/Switch A Circuit High
- P0844Transmission Fluid Pressure Sensor/Switch A Circuit Intermittent
- P0845Transmission Fluid Pressure Sensor/Switch B Circuit
- P0763Shift Solenoid C Electrical
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.