Engine ckeck light has come on!

This is the problem that has beaten my garage.
Car-Accent 1.6 CDX 3 years old
The engine check light has come on twice in three days and flashed for
about 15 secs each time. Both times it occurred after just over a minutes driving from cold. The garage checked the error messages and it said Random misfire on cylinder one, and on two, and on three, and on four.
They took the car in for days and checked everything that could be checked, but came up with the answer that all was perfectly in spec and that nothing was wrong.
Can anybody help by suggesting what could be wrong with the engine. Any serious sleuthing would be very welcome.
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Intermittent problems are difficult to diagnose. Misfires can happen from ignition parts starting to go bad. Modules, wires, etc. Damp and cold weather magnifies that sort of thing. Have the plugs been changed? Wires? If nothing else crops up, they are cheap places to start.
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including leads and plugs etc (which were changed 5,000 miles ago) and found absolutely nothing. They concluded it might be mechanical but couldn't find anything. Any more ideas?

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Since in both cases it happened from cold, make a note of the temperature outside. It may only do so, say, below 30 degrees. If that continues, leave the car at the shop on a cold day and have them put the meter on it when they start it up. I can't think of many mechanical items that can cause a mis-fire. Since it happens on all cylinders, it is more likely to be something like an ignition module than an individual wire. Engines have many sensors also and one may be malfunctioning giving a bad signal, injection problem, etc.
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Since you have misfire readings on all four cylinders, you should start looking at things that affect all cylinders equally, such as fuel pressure, fuel quality, and perhaps even the crank sensor.
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