P0843
PowertrainTransmission Fluid Pressure Sensor / Switch "A" Circuit High
The gearbox control module keeps an eye on the voltage coming back from the transmission fluid pressure sensor or switch on circuit 'A'. When that voltage sits higher than the module expects for the conditions it's seeing, it logs P0843. For you as the driver, that usually points to an electrical gremlin in the sensor or its wiring rather than a genuine pressure fault inside the box, though it can be either.
ⓘ Information only. This page provides general educational information about fault code P0843. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0843 mean?
P0843 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Transmission Fluid Pressure Sensor / Switch "A" Circuit High.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine management or 'Check Transmission' light on the dash
- • Harsh, delayed or hunting gear changes, often worst when the gearbox is cold
- • Box dropping into limp mode and holding a single gear, usually third
- • Slipping or a refusal to engage certain gears under load
- • Flashing 'D' light on the selector on some Honda automatics
- • Jerky, snatchy shifts when you accelerate hard
Possible causes
Causes commonly associated with P0843, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Faulty fluid pressure sensor or switch on the gearbox casing, the usual suspect and often the cheapest fix
- 2. Corroded or water-damaged connector at the sensor, common on cars that see a lot of motorway salt and road spray
- 3. Chafed or shorted wiring in the harness near the bellhousing where heat and movement wear the insulation through
- 4. Old, burnt or low transmission fluid throwing the sensor readings off, particularly on boxes that have never had a fluid service
- 5. Loose or half-clipped sensor plug disturbed during a recent service or sump job
- 6. Failing TCM or PCM, less likely but it does happen on higher-mileage cars
- 7. Internal hydraulic issue or a blocked feed, the rarest cause and the most expensive to chase
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Scan the gearbox module for live data and all stored codes, then look at the actual pressure sensor voltage against what the box is doing. A reading pinned at or near the 5V reference with the engine running tells you a lot
- 2. Unplug the sensor connector and inspect both halves for green corrosion, bent pins or moisture, then check the plug clicks home properly. A dirty or pushed-back terminal causes this more than people expect
- 3. Back-probe the signal and reference wires with a multimeter, ignition on and engine off, and compare against the manual. You're looking for a sensible 5V reference and a signal voltage that isn't sitting maxed out
- 4. Check the fluid level and condition on a level surface at the right temperature. Dark, varnishy or burnt-smelling fluid means a service is overdue and can be skewing the reading
- 5. Wiggle-test the harness along its run to the sensor while watching live data on a test drive, since intermittent shorts near heat sources only show up when everything's warm and moving
Common questions about P0843
Is a budget sensor off eBay any good, or do I stick with the genuine part? +
For the pressure sensor itself a quality branded aftermarket part is fine and saves you a fair bit over the dealer price. The trap is the no-name £15 specials, which can read slightly off and have you back chasing the same code in a fortnight. Stick to a recognised brand, and on anything with a fussy TCM such as later Hondas and ZF boxes, a genuine sensor is the safer money. Whatever you fit, top the box up with the correct spec fluid afterwards.
Can I keep driving it like this? +
Short trips to get it looked at are usually fine, but I wouldn't rely on it. Once the box is acting on a bad pressure signal the shifts get unpredictable, and if it drops into limp mode you're stuck in one gear, which is no fun pulling onto a motorway or a roundabout. Get it diagnosed before the dodgy shifting starts cooking the clutch packs, because that turns a cheap sensor job into a gearbox bill.
Will this fail my MOT? +
The code on its own isn't an MOT item, gearbox faults aren't part of the test. What can catch you out is the engine light being on at the time of inspection, which the tester has to record. Fix the cause, clear the code and drive a few cycles so the light stays off before you book it in.
What am I looking at to put it right? +
If it's just the sensor, an independent garage will usually have you out the door for a low three-figure sum with parts and labour, often less if it turns out to be a connector clean or a wiring repair. A main dealer will charge noticeably more for the same job. Where it climbs is when the fault is internal to the box or a failed control module, which can run into the high three figures or beyond once labour and programming are added.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →