P0840: Transmission Fluid Pressure Sensor/Switch A Circuit
What to do
If this code is stored alongside slipping or a gear ratio fault, say so — that combination points at hydraulic pressure rather than at worn clutches, and those are very different repairs. Have the fluid level and condition checked first, since low fluid produces low pressure and is the cheapest explanation on the list.
What this code means
A fault in the circuit monitoring hydraulic pressure inside the transmission. This code family matters more than its position in the numbering suggests, because pressure is the thing the whole transmission block has been pointing at.
Throughout P0700–P0799 the recurring instruction is have the line pressure measured with a gauge — under P0730, under P0745, under P0746. These codes are the vehicle doing that measurement itself. A pressure fault stored alongside a gear ratio code or a slipping complaint is therefore not a second problem to investigate; it is corroboration, and it moves the diagnosis decisively away from worn clutches and towards the pump, the filter and the valve body. That is a large difference in cost, and it is available for free from a code that is already stored.
"Sensor or switch" is not vagueness in the code name — the two are genuinely different parts and they determine what is possible diagnostically. A switch is a threshold device: it closes when pressure passes a point, and the module learns only whether that point was reached. A sensor reports an actual value, so the module can compare pressure against what it commanded across the whole range. Establishing which is fitted is the first step, because a switch can only ever answer a yes-or-no question and no amount of scan data will make it answer more.
Likely causes
Start here
- Low or degraded transmission fluid · easy
- Faulty transmission fluid pressure sensor · moderate
- Damaged transmission wiring harness · moderate
Less common
- Restricted transmission fluid filter · moderate
- Worn valve body · specialist
- Worn transmission fluid pump · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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