96 Cadillac Seville STS problems-?

takes about three times to start up just had a full tune up, oil change and catalytic converter replaced and now is having this issue i replaced the coil packs cleaned & gapped the spark plugs (they are new but seem to have a lot of oil) also there is a rich smell from the tail pipe when it does start up. Im... show more takes about three times to start up just had a full tune up, oil change and catalytic converter replaced and now is having this issue i replaced the coil packs cleaned & gapped the spark plugs (they are new but seem to have a lot of oil) also there is a rich smell from the tail pipe when it does start up. Im thinking to replace the O2 sensor and if that doesnt work I read the fuel injectors may be faulty any other ideas what it could be? Thanks
Update: no check engine light either btw only change oil light is on but it was just changed about one month ago and never went off
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