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Error Code
E3 means the high pressure switch has tripped. Pressure inside the system climbed past its safe limit and the switch opened to protect the compressor, so the unit shut down. The switch is doing its job, and the fault is whatever pushed the pressure up. In practice that is restricted airflow across a coil, an overcharge, or a restriction in the refrigerant circuit, and the airflow checks come first.
A condenser clogged with leaves, pollen or pet hair cannot reject heat, so pressure climbs until the switch opens. The indoor coil matters just as much when the unit is heating. Cleaning both coils is the cheapest test on this page and it resolves a large share of E3 codes, particularly on units that have not been serviced in a few years.
A system short of refrigerant can run high pressure as well as low, because the compressor has less gas to work with and the coil temperatures get out of step. It sounds counterintuitive after a low pressure code, which is why the charge is measured rather than assumed. Any shortfall means a leak that has to be found.
The switch or sensor itself can fail and report pressure that is not there. This is the diagnosis of elimination: once the coils are clean, the charge is correct and the unit still trips, the sensor becomes the likely cause. Replacing it before checking actual pressure is how a healthy system ends up with a new part for nothing.
A blocked filter drier, a kinked line or a partly closed valve holds pressure on one side of the restriction. The giveaway is a temperature difference across a component that should not have one. Finding the restriction is a measurement job, and doing it stops a pressure fault being blamed on the switch.
The return filter comes out first, then the indoor coil is checked for dust matting and the outdoor coil for leaves and grass. Both are cleaned before any gauges are connected. On an E3 this is not just good practice, it is the most likely fix, and it costs nothing but time.
Charge is confirmed by weight or by subcooling and superheat rather than by a single pressure reading, because pressure moves with outdoor temperature. If the system is short, the leak gets found before any gas goes in. Topping up a leaking system only delays the next E3.
With the system isolated, the switch is tested for continuity and the sensor reading is compared against a gauge on the same port. A switch that opens at the wrong pressure, or a sensor that does not track the gauge, has failed. This is how a false E3 gets caught before a part is ordered.
Isolating the unit for a few minutes lets the pressure equalise and the switch reset. Watch what happens next: an immediate trip means pressure is genuinely high, while twenty minutes of running before the trip suggests a coil or airflow problem building heat. The reset is diagnostic only, so do not repeat one that trips straight away.
