Go to any mechanic with any sort of live scan device, warm up the engine and see the following items
1 - Short term fuel trim
2 - long term fuel trim
3 - spark timing advance
4 - AFR sensor reading
5 - post cat sensor reading
If you are seeing long term trims nearing about more than 10% in anyway (+ve or -ve) then you need to sort out why this is happening. To see it the AFR sensor is very a nice indicator to see the fuel mix, make sure the MAF is also seeing something going. If your trims are cuckoo a systematic approach is required to bring the tune back to factory specification.
In some cases AFR sensors have worn out (unlikely but possible) - missing cat will cause problems as the ECU sees the post cat o2 sensor wildly fluctuating (meaning no or dead cat present) - fuel leak off into the manifold also causes excessive fuelling