EngineLightCodes
Powertrain code

P0843 Transmission Fluid Pressure Sensor/Switch A Circuit High

Severity
Fix soon
Typical repair cost
$130–$990
Do it yourself
Shop job
Shop time
1–5 hours

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. 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.

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

  1. Signal or ground wire open, connector soaked with fluid Most likely
  2. Switch stuck open or sensor faulty Most likely
  3. Signal wire shorted to 5 V or battery voltage Possible
  4. Too little pressure at the switch: low fluid level or sticking valve Possible
  5. Control module faulty Less likely

How to diagnose P0843

  1. 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.
  2. Inspect the parts involved: transmission fluid pressure sensor, transmission valve body, transmission wiring, transmission control module.
  3. Work through the causes above from most to least likely, cheapest checks first.
  4. 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
43
Specific fault

Questions drivers ask

Can I drive with code P0843?

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 P0843?

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.