I have same problem when i drive my 00 fiat seicento everyday its fine but if i leave it 1 or two days it wont start but yellow charge light comes on ????

Question edited for clarity.

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code p 0300 refers to random/multiple cylinder misfires detected==causes spark plug/s ---HT lead/s--injector/s--ignition coil/s-low compression --- wiring
it could be simple moisture in the distributor cap, wiring connections
It could be a problem ECM unit
monitor the condition or take it to an accredited service center that has experience on your make of vehicle and have it checked out
could be a problem that has already been identified and has a manufacturer service bulletin in place

Technical: HT Leads diagram? - The FIAT Forum

www.fiatforum.com > FIAT Cars > Cinquecento / Seicento
Aug 30, 2008 - 9 posts - ‎3 authors Anyone got a simple diagram of where each lead goes from each plug to which ... Makesure the HT leads are on correct, then try them either way. ... As long as 1+4 spark plugs go to one coil pack, then 2+3 go to the other coil ... Fuel light flashing but won't start (No spark)
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ignition - Methods of diagnosing a non-starting Fiat Uno ...

catalylic converter

If your Fiat has no distributor, a distributorless ignition system, then it will have a coil pack with two coils. Each coil provides spark for two cylinders. One coil is for cylinders 1 and 4, and the other coil is for cylinders 2 and 3. If one of your coils has quit working, you will only be running on 2 cylinders. It is not uncommon for coils to fail.

This may be a puzzle...
The seicento (I drive the same car) likes to create problems in the heater section of the cooling system. The small pipes to it (behind the carburator) may be blocked or the entire heater may simply be dirty. Flushing the system with a water hose may help. Usually this problem starts slowly and goes away if you re-start the engine and then excellerate a few times. Check if your heater blows hot air - if yes, and if he keeps blowing hot air (does not cool down and get cold) ther is no blockage. If you have a blockage, your heater will only blow hot for a little while and then comes normal cool air - if this is the case, you have a blockage right in your heating system which runs your cooling system hot and the cap of your cooling tank will start leaking. This is a comon problem on older Seicentos....
Other general problems can also be the case, like defective thermostat, fan motor does not come on (check fuse and relais first).
In some cases, ther might be some air trapped in the cooling system and doing the above will also cure it. At last, check if your water pump is working.
Regards
Herbert

you have tripped the inertia switch, (there to cut off the fuel pump in the event of a collision), you need to reset the switch by simply pressing it back in.

check fuses first, if all fuses ok then u have bad cig lighter.

You should get your TDC sensor checked. TDC sensors can play funny tricks and get you off track very easily.

it could be a possibility that it has a faulty rpm sensor have been known to go.