How to check a used air conditioner before you buy
6 min readUpdated
How to check a used air conditioner before you buy
Run the unit for at least fifteen minutes and check five things: that the air leaving the indoor unit is genuinely cold, that the compressor in the outdoor unit starts and holds, that the pipes and coils show no oily residue, that water drains away cleanly, and that the remote controls every function.
Why a fifteen-minute test matters more than anything else
Almost every serious fault in an air conditioner shows itself within the first fifteen minutes of running. A unit that has been switched on shortly before you arrive will feel cold no matter what condition it is in, because the initial burst of cooling comes from refrigerant already sitting in the coils. The problems appear once the compressor has had to cycle.
Ask for the unit to be off when you arrive, then start it yourself and keep it running while you work through the rest of the inspection. Anything that fails after ten minutes — cooling that fades, a compressor that cuts out, a breaker that trips — is exactly the fault a seller hopes you will not wait around long enough to see.
This applies to split units and window units alike. Inverter models take slightly longer to settle into their steady state, so give those the full fifteen minutes rather than cutting the test short.
The five checks that matter
Work through these in order. The first two eliminate most bad units on their own.
Cooling at the vent
Hold your hand at the indoor unit's outlet. After ten to fifteen minutes the air should be distinctly cold, not merely moving. If it starts cold and weakens, suspect low gas or a failing compressor. A thermometer at the vent is better than a hand if you have one — you are looking for a clear difference between intake and outlet air.
Compressor behaviour
Go to the outdoor unit. The compressor should start within a couple of minutes and run steadily. Listen for loud rattling, grinding, or a start-stop pattern every few minutes. Short-cycling is a serious symptom and usually expensive. Some vibration is normal; a unit that shakes its mounting is not.
Pipes, coils and joints
Look for oily film or dark residue around the copper pipes and their joints. Refrigerant carries compressor oil, so an oily patch marks a leak even when the unit currently cools. Check that the insulation on the pipes is intact rather than crumbling, and that the fins on both coils are not crushed flat across large areas.
Drainage
Once the unit has run a while, water should drain steadily from the drain pipe. No water at all can mean a blocked line, which shows up later as water dripping down an interior wall. Water appearing anywhere other than the drain outlet is a fault, not a quirk.
Remote and electricals
Cycle through every mode, change the temperature, change the fan speed and swing setting. Confirm the display responds and the unit obeys. Check the plug and cable for scorching or taped repairs, and ask whether the circuit trips when the unit starts.
What the outdoor unit tells you about age and care
The outdoor unit lives outside and records how the machine has been treated. Heavy rust on the casing usually means it sat exposed with poor drainage around it. A thick mat of dust and leaves in the coil means it was never serviced, which forces the compressor to work harder for years.
Look at how it was mounted. A bracket that has sagged, or feet that are not level, puts the compressor under constant strain and is a reasonable sign the installation was done cheaply. None of this is automatically disqualifying, but it should shape what you expect to pay and how soon you budget for a service.
The rating plate on the outdoor unit gives you the model number and often the manufacturing date. That is worth photographing: it lets you check the original specification later rather than relying on what you were told.
Common symptoms and what they usually mean
None of these are certainties, but each points at a likely cause worth investigating before you agree a price.
| What you notice | Likely cause | How serious |
|---|---|---|
| Cools at first, then weakens | Low refrigerant, often from a slow leak | Moderate — needs leak repair, not just a refill |
| Outdoor unit starts then stops repeatedly | Compressor or capacitor fault, or overheating | Serious — frequently the most expensive repair |
| Oily residue on pipe joints | Refrigerant leak | Moderate to serious depending on location |
| Water dripping inside the room | Blocked or misrouted drain | Minor — usually a cleaning job |
| Loud rattle from the indoor unit | Fan blade or bearing wear | Minor to moderate |
| Breaker trips on start-up | Electrical fault or undersized circuit | Serious until diagnosed |
Questions worth asking the seller
Answers matter less than whether they are consistent and specific.
- How old is the unit, and do you have the original purchase receipt or box?
- When was it last serviced, and by whom?
- Has the gas ever been refilled? If so, how many times and why?
- Has any part been replaced — compressor, fan motor, PCB?
- Why are you selling it?
- Is the outdoor unit accessible for me to inspect properly?
Buying from a dealer instead
The checks above assume you are inspecting the unit yourself, which is the right approach when buying privately. Buying from a dealer shifts that work onto them: at IslamabadSalePurchase every unit is inspected before it is listed and carries one of four published condition grades — New, Like New, Good Used, or For Parts — so you know what you are looking at before you make contact.
Eligible used appliances also come with a 15-day checking warranty from delivery, which exists precisely because a fifteen-minute test in someone's living room cannot tell you everything. It gives you a window to run the unit properly in your own home and raise anything that does not match the description.
That does not make inspection pointless. Ask which grade a unit carries and why, and ask what was serviced before it was listed — a dealer who cannot answer that has not really checked it.
Operator input needed
Add the two or three faults your team most often finds on units brought in around Islamabad and Rawalpindi — that first-hand detail is what makes this guide worth citing rather than generic.
Frequently asked
How long should I run a used AC before deciding?
Is an oily patch near the pipes always a leak?
Should I buy a used inverter AC or a used conventional one?
Does a gas refill fix a unit that is not cooling?
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Tell us the item and budget and we will check what we have and what we can source.
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