Engine light

Hi everyone again!!
 
Had my engine light on last week, someone told me to remove fuel cap and put back on to solve, this did work for a week or so and now light back on, tried that trick again but never worked this time so light remains on.
Was this a coincidence or not. Should I get a a new cap ordered anyway?

Get a new OEM cap.  Happens all the time

OK i have ordered one today.
But one more worry is that when light came on there was a tempory loss of power, it did it again this morning then light comes on, surely this is unrelated to the cap???

It can be.  The loss of pressure in the fuel system can do that.

ok that makes sense.
Anyway I have played with cap and replaced this afternoon, light went out, bingo!
BUT while engine running i removed cap and did a little drive with cap of, light never came up!!
When I took cap off(with engine running) there is no sign of pressure at all in the tank, so now i dont understand??
 

I have had the same problem. This is what my SAAB mechanic said. When you start your engine, one of the checks that the system does is of the Fuel Evaporation System. It does so by checking the pressure in the system. If the pressure is TOO low, it assumes that there is a leak and turns on the CEL with code P0455. This code is for vacuum leak (large). Since it does the test at start, removing the gas cap while it is running will not necessarily trip it. There are other possible causes; faulty purge valve, bad hoses, etc. Bad gas cap is most common and by far easiest to remedy.

Exactly correct.  This diagnostic is only run at startup when the global system check runs.