Reads codes of po300,po302,po303,po305 - 2003 Cadillac CTS

P0300-Random/multiple cylinder(s) - misfire detected
P0302- Cylinder 2 - misfire detected
P0303- Cylinder 3 - misfire detected

Do tune up,coils module,wires,plugs and see what happened..

If a tune up (spark plugs, ignition wires, distributor cap/rotor button or coil packs) don't work, try checking the fuel system.

We can't help you here

You need the car diagnosed

I doubt you have lifters,should have
OHC valve train

Most likely the oil passages are clogged up
in the cams with sludge

That's what happens when the wrong oil
& too long between changes takes place

You ruin the engine

PO171 OBD2 Code basically this means that an oxygen sensor in bank 1 has detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 1 is the side of the engine that has cylinder #1. The P0171 is one of the more common trouble codes.

his code is triggered by the first downstream (front) O2 sensor. The sensor provides a reading of the air:fuel ratio leaving the engine's cylinders, and the vehicles powertrain/engine control module (PCM/ECM) uses that reading and adjusts to keep the engine running at that optimum ratio of 14.7:1. If something is not right and the PCM cannot maintain the 14.7:1 ratio, but rather there is too much air, it triggers this code.

Ignition failure sensor circuit problem. Either a bad sensor or connection.

Sounds like you still have air leak.

These are cylinder misfire codes meaning ;
P-0300 = RANDOM OR MULTIPLE MISFIRE
P-0302 =CYLINDER #2 MISFIRE
P-0305 =CYLINDER #5 MISFIRE
Check spark plugs, wires, and coil(s)

Hey Dan, It cannot hurt to change the Plugs and wires. Make sure and Check all the Fuses and Relays also. Sometimes the Realays can blow or just get old and send a Code also.

Try some scrapdealers. They keep bits they think they can sell from crashed/scrapped cars etc.