check engine light intermittant. Buick Park Avenue 1989

Hi,
the subject says most of the problem.
I bought at auction locally a 1989 Buick Park Avenue 3.8 v6 with 150,000 miles.
Car looks excellent , some rust on bottom of doors but better than
expected for age and milage.
The 'check engine' light comes on intermittantly but all gauges show normal and little apparent change in performance.
Using a paper clip on the diagnostic connector (after I found it) got me flashing lights of 12, 13 and 48. The 48 does not show on the codes I got off the internet and may be incorrect.
Code 58 may be right because I do not have any sort of radio key. I will check this again later.
I understand code 12 is a check code to say the system is working - ignore it.
Code 13 says something about O2 sensor voltage blah blah blah.
What is the most likely fault and fix.
replace the O2 sensor ? new air filter ?
thanks for any suggestions.
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checked the codes again and is definately code
12 - 13 - 48
13 is o2 sensor problem 48 on one site relates to misfire detection
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O2 on that car is 25.00, throw one in.

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