Check Engine light on Suburu Forester 2002

I brought a 2002 Suburu Forester in Dec 2019. The day after I took it home the
check engine light went on, so I took it back to the car yard and they did a
scan, said something about oxygen and the light went off, but by the time I got
home it was back on. Few weeks later I had the car serviced, but they found
no codes, but did find $1500 worth of repairs and over the last few months I've
had them fixed (new top radiator hose; new front filter; rear oil seal leak; wheel
alignment, right hand inner drive shaft; new brake fluid; plus the drive belt
snapped and that was replaced.

The last repair was the rear oil seal leak. After I collected the car the check
engine light was off. But after 4 drives (all around 4km each) it came back on. I
took it back to the same mechanic who still couldn't find any codes, infact, it
wasn't even coming up, though 2003 came up but no error codes were found.
They have suggested fitting new oxygen sensors which is going to cost me
around $700. This is a 'general' mechanic, not a Suburu specialist.

I'm concerned this may not fix the issue and is a waste of money. Could it be
something else? Why would the light go off and then come back on? Could it
just be a faulty light? Why aren't they getting any codes? And would a Suburu
mechanic have a different scan tool (but cost more regarding any repairs)?