Back in May, I replaced the Cat and all 3 O2 sensors in my 2003 4.3. Now I keep throwing random misfire and O2 sensor 1 codes (P0300, P0131,P01710,& P1133). I also replaced the vacuum line from the intake to the HVAC. I do have good vacuum now. Any ideas?

 

is the third code P0171?

P0300 is random misfire as you have identified
P0131 O2 sensor low voltage bank 1 sensor 1
P0171 System too lean bank 1
P1133 Heated Oxygen Sensor (HO2S) Insufficient Switching Bank 1 Sensor 1

From that I would guess you have bad wire or bad O2 sensor on Bank1 sensor 1 which would give some misfire. I forget which one that is but here on the board the information is available.

Did you use Bosch O2 sensors? Some people had reported issues so I would recommendan ACDelco or Denso.

 

they are Denso and I've already replaced that one once. I guess its a bad wire