Trouble starting 9-5 2.4t auto

I could not get my car started when I came out of Tesco I had previously done a few short trips and had left my lights on the night before for a few hours so assuming it was a flat battery I got a jump start and all was fine for a week untill again short trips and no start this time outside my house so I took the battery out charged it and all was fine thinking my battery was dead I ordered one and next time it happened I just quickly changed battery's over but this didn't fix it still not starting it would turn over but not fire after quite a few cranks it fired up only to drop revs and died then it started straight away but again the revs dropped down put picked up and all was fine I'm not getting any faults coming up on dash if it's not my battery what else could it be

Might be the Crank position sensor.

Would that not show as a fault

I don’t think so. I’ve read many posts where cp’s have failed with no warning light.

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