Galant will not start

2002 Galant which sometimes starts and sometimes won’t for no apparent reason …it started a few weeks back he would start it and drive great then return to the car and it wouldn’t start so he would leave it for 1 hr then all good again ? but now it struggles to start at all …and often won’t ? any ideas ?

You really need to define no-start a bit better. It helps to narrow the possibilities down.

Engine does not crank at all? No noises or clicks at all? Starter motor cranks the engine over slowly or in a normal manner? Cranks over fine but no coughing, sputtering, etc? Automatic or manual transmission?

Ok , sorry …it just clicks most of the time and sometimes sounds like it may catch but doesnt at the last second. It is auto .
From reading different comments on different sites this seems to be a common fault but nobody has a firm answer to what it is , some people have changed batteries / starters/ o2 sensors . Any help would be great .
Thanks

Remove the 02 sensor from your list of items that could cause a no crank,no start condition, it is still not clear if we are dealing with a no/intermittent crank or a crank no start condition.

If by clicking you mean the started clicks but does not crank then it could be the starter. If it cranks but does not start then we have a different list.