P0840: Transmission Fluid Pressure Sensor/Switch A Circuit
P0840 is a powertrain diagnostic trouble code in the transmission group. Our vehicle database stores it against 15 models.
How serious is P0840?
We rate P0840 as High — address promptly, the top of our three-level scale. That rating comes from the severity recorded in our database for this code, not from how alarming the name sounds.
A high-severity code means the fault can damage expensive components if it is ignored. Continuing to drive is a judgement call that depends on how the vehicle behaves: if it is misfiring badly, losing power, overheating, or flashing the check-engine light, stop driving and have it recovered. A flashing check-engine light specifically indicates an active misfire that can destroy a catalytic converter within a short distance.
What P0840 actually means
P0840 is worth reading before the next drive. It is a powertrain code in the transmission group, and our database rates its severity as high. 15 models in our database log it.
The stored definition for P0840 is “Transmission Fluid Pressure Sensor/Switch A Circuit”. The first character of the code identifies the system — Powertrain in this case. The second character tells you who defined the code: this one is a generic code defined by SAE J2012 and shared by every manufacturer. Reading the code that way matters: a generic code tests the same underlying condition on almost every vehicle, so the causes below apply broadly across makes.
Within that system, P0840 sits in the transmission group. That is the part of the vehicle the control module was watching when it set the code — the transmission’s valves, solenoids, clutch packs, and fluid circuits — and it narrows down enormously where a technician will start looking. Transmission codes are read alongside line pressure and slip data, because the same code can come from a stuck valve or a worn clutch.
Most likely causes of P0840
Our database records a single cause for P0840, reported by the 15 models that log it.
CVT pressure sensor failure — recorded on every model that logs this code.
Causes are listed as recorded in the database. The same code can have different root causes on different vehicles, so the ranking is a starting order for diagnosis rather than a diagnosis in itself.
How a technician diagnoses P0840
Diagnosis for P0840 follows the same logic as any stored powertrain code: confirm the code is active rather than historic, then work through the ranked causes from cheapest and most likely to most intrusive. In practice that means:
How a technician works P0840 — decision path
Powertrain codes respond to data. Capture the freeze frame, then work through the ranked causes from cheapest and most likely to most intrusive.
- Confirm the code is active and capture freeze-frame dataThe snapshot recorded when the code set holds the engine speed, load, coolant temperature and fuel trims at that moment — often the single most useful piece of evidence for separating one candidate cause from another.
- Compare live data against the freeze frameRe-create the conditions from the snapshot and watch the relevant sensors live. A value that cannot reach the expected range narrows the search immediately.
- Work the ranked causes from cheapest and most likelyThe list on this page is ordered by how many of the recorded vehicles show each cause. Start at the top: an intake leak, a dirty sensor or a worn cap is far more common than a failed assembly.
- Repair the confirmed cause, then re-check the monitorsFix what the evidence actually points at rather than replacing parts speculatively — a misfire or lean code will re-set if the real cause is still present.
- Clear the code and complete a full drive cycleClearing alone proves nothing. The code must stay clear through a drive cycle that reproduces the original conditions, and the readiness monitors must run again.
If a workshop replaces a component for P0840 without showing you the measurement that condemned it, ask for the data. On a transmission code, the recorded causes are usually testable, so a diagnosis should be demonstrable rather than asserted.
What drives the cost of fixing P0840
We deliberately do not print a single repair price for P0840. Transmission work is the expensive end of the range once the cause is internal rather than a solenoid or fluid issue. The honest answer is that the cost is set by which cause is actually present, and that range is wide — a repair for the first cause listed above and the last can differ by more than an order of magnitude in parts alone: a solenoid and a clutch pack are not the same job. Add regional labour rates and the spread becomes wider still.
The factors that actually move the number, in rough order of impact:
- External or internal. A solenoid, fluid, or connector fault is one bill; a valve body or clutch pack is a different order of magnitude.
- Fluid and filter service. On neglected units a fluid and filter change is a legitimate first repair and the cheapest way to test the hypothesis.
- Removal labour. When the cause is internal, the transmission comes out — and that labour, not the parts, is usually the largest line.
- Shop specialisation. Transmission work is the group where shop choice most affects both the rate and the risk of a re-do.
The practical approach: get the cause confirmed first, then collect two or three written quotes for that specific repair. To judge whether the repair is worth doing against the value of the vehicle, use our repair-or-replace decision guide.
Vehicles in our database that log P0840
15 models store this code. The makes appearing most often are Nissan (5), Mitsubishi (4), Buick (3), Chevrolet (2), Hyundai (1). Each model links to its own guide, where this code appears alongside that vehicle's full fault list and running costs.
Frequently asked questions about P0840
What does the P0840 code mean?
P0840 is a powertrain trouble code meaning "Transmission Fluid Pressure Sensor/Switch A Circuit". It belongs to the transmission group of the SAE J2012 numbering scheme, and it is a generic code defined by SAE J2012 and shared by every manufacturer. The code is set by the powertrain control module when the monitored system reports a value outside its expected range.
Is it safe to drive with P0840?
Severity for P0840 is rated high in our database. A high-severity code means the fault can damage expensive components if it is ignored. Continuing to drive is a judgement call that depends on how the vehicle behaves: if it is misfiring badly, losing power, overheating, or flashing the check-engine light, stop driving and have it recovered. A flashing check-engine light specifically indicates an active misfire that can destroy a catalytic converter within a short distance.
What are the most common causes of P0840?
In our vehicle database, the causes recorded for P0840 are: CVT pressure sensor failure. They are ranked on the page by how many of the 15 vehicles that log this code list each cause.
How much does it cost to fix P0840?
We do not publish a single figure for P0840 because the real cost depends entirely on which cause is present — a repair for the cheapest cause on the list and the most expensive one differ by an order of magnitude, and labour rates vary by region. The page explains which factors drive the cost and recommends collecting written quotes before authorising work.
Which vehicles commonly report P0840?
Our database stores P0840 against 15 vehicles, most frequently from Nissan (5), Mitsubishi (4), Buick (3), Chevrolet (2), Hyundai (1). The full list is shown on this page and each model links to its own cost and fault guide.
Related codes and next steps
Codes that share the same system group often appear together, because a single fault can set more than one:
For the full list, see the OBD-II fault code cross-reference. If the repair cost is what you are weighing, model the total with our total cost of ownership calculator before deciding, and check whether an open safety recall covers the fault with the recall lookup.
Sources and scope
Code definitions follow SAE J2012 / ISO 15031-6 naming. The descriptions, listed causes, and severity ratings on this page are the values stored in the VehCalc vehicle database (snapshot 2026-08-20) for the models listed above. Codes and causes vary by model year and engine.
Diagnostic reference only — not repair advice. This page is provided for educational and planning purposes and does not replace an inspection by a qualified technician or the manufacturer's service information. Before repairing a vehicle you depend on, confirm the diagnosis with an ASE-certified mechanic.