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. P0843 means the signal is too high. For a switch that is 'open although pressure should be there': Allison sets it when switch 1 stays without pressure for over 0.07 s with valve S1 applied, type A; a 2006 GM four-speed used switch A for the converter clutch release and set it after 6 s, type B. For a sensor it is a short to voltage or open: GM's CVT above 95 %, its 8-speed above 91 % duty cycle.
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
- Signal or ground wire open, connector soaked with fluid
- Switch stuck open or sensor faulty
- Signal wire shorted to 5 V or battery voltage
- Too little pressure at the switch: low fluid level or sticking valve
- 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 P0843 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 8: Transmission
- 43: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0843?
Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
What causes code P0843?
The most common causes are: signal or ground wire open, connector soaked with fluid; switch stuck open or sensor faulty; signal wire shorted to 5 V or battery voltage.
Related codes
- P0840Transmission Fluid Pressure Sensor/Switch A Circuit
- P0841Transmission Fluid Pressure Sensor/Switch A Circuit Range/Performance
- P0842Transmission Fluid Pressure Sensor/Switch A Circuit Low
- P0848Transmission Fluid Pressure Sensor/Switch B Circuit High
- P0741Torque Converter Clutch Circuit Performance/Stuck Off
Sources
- gsi.ext.gm.com: 23OBDG03A Part1
- gsi.ext.gm.com: 25OBDG06A
- gsi.ext.gm.com: 07 GRP15 Allison 1000
- gsi.ext.gm.com: 2006trans5 b
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.