2001 Caddie STS trouble code P0308 which is No. 8 cylinder which is probably plugged in the injector I suspect.

Those are misfire codes, so those cylinder have some kind of issue like defective ignition coils or fuel injectors. You will need to check all that.

Low cylinder pressure, defective fuel injector or fuel injector wiring harness, defective Powertrain control computer, intake manifold leaks vacuum.

Code 301 means cylinder one is not producing power.
Problem could be ignition, fuel, or mechanical.
If compression is good on all cylinders, check the fuel injector.

sometimes the fuel injector can be out of range but this would give a trouble code.
then check spark plug to see if it is wet or fouled.
check plug wire for external sparks.
last check the coil pack.

Trouble Code: P0302

Cylinder 2 Misfire Detected

Possible Causes:


Air leak in the intake manifold, or in the EGR or PCV systemBase engine mechanical fault that affects only Cylinder 2Fuel delivery component fault that affects only Cylinder 2 (i.e., a contaminated, dirty or sticking fuel injector)Ignition system problem (coil or plug) that affects Cylinder 2

Trouble Code: P0300

Multiple Cylinder Misfire Detected
Possible Causes:


Air leak in the intake manifold, or in the EGR or EVAP system Base engine mechanical fault that affects two or more cylinders Erratic or interrupted CKP or CMP Sensor signals Fuel delivery component fault that affects two or more cylinders (i.e., a contaminated, dirty or sticking fuel injector)

Trouble Code: P0302

Cylinder 1-10 Misfire Detected
Possible Causes:


Air leak in the intake manifold, or in the EGR or EVAP system Base engine mechanical fault that affects only one cylinder Fuel delivery component fault that affects only one cylinder (e.g., a dirty fuel injector)

Trouble Code: P0305

Cylinder 1-10 Misfire Detected
Possible Causes:


Air leak in the intake manifold, or in the EGR or EVAP system Base engine mechanical fault that affects only one cylinder Fuel delivery component fault that affects only one cylinder (e.g., a dirty fuel injector)

Trouble Code: P0351

Ignition Coil No. 1, 2 or 3 Primary Circuit
Possible Causes:


ASD relay power circuit to the ignition coil is open Ignition Coil No. 1, 2 or 3 primary "driver" circuit open or grounded Ignition Coil No. 1, 2 or 3 is damaged or it has failed

Trouble Code: P0352

Ignition Coil No. 1-8 Primary Circuit
Possible Causes:


ASD relay power circuit to the ignition coil is open Ignition coil primary "driver" circuit open or grounded

Trouble Code: P0353

Ignition Coil No. 1-8 Primary Circuit
Possible Causes:


ASD relay power circuit to the ignition coil is open Ignition coil primary "driver" circuit open or grounded Ignition coil is damaged or it has failed PCM has failed Ignition coil is damaged or it has failed PCM has failedPCM has failed Ignition system problem (coil or plug) in only one cylinder Ignition system problem (coil or plug) in only one cylinder Ignition system problem (coil or plug) in two or more cylinders Vehicle driven with low fuel pressure or while very low on fuel

You've done a great job of trouble shooting thus far. I would certainly suspect the injector after all of that. But, I didn't see anything about you replacing the spark plug itself. I would try that first as a crack in the insulator (even hairline) will cause mis-fires. If the spark plug doesn't fix the mis-fire, it cold be caused by a bad fuel injector, bad signal (like a loose harness or bad wire) to the fuel injector, or a leak or valve issue on that cylinder.

A po300 means you have several cylinders misfireing ! Depending on mileage it could just need new plugs ! If you try new plugs only use delco plugs in that vehicle ; it will save you a lot of problems later down the road !

All these codes indicate you have a misfire on cylinder number 5.
P1399/P0300 - Random/Multiple Cylinder Misfire Detected
P0305 - Cylinder #5 Misfire Detected

Causes:
- Improper spark plug at cylinder #5
- Insufficient compression at cylinder #5
- Incorrect fuel pressure
- The injector circuit is open or shorted at cylinder #5
- Fuel injector #5 may be faulty
- Intake air leak at cylinder head

Solutions:
- Perform Injector Service
- Replace Spark Plug #5
- Replace Injector #5
- Replace Ignition Coil #5

I have a 2001 STS that had the same problem - Service Engine light and returned the P0410 code. It ended up being a small rubber hose that had pulled off one of the plastic lines in the secondary air system. In fact, I just fixed this last night.

Pull off the top engine shield (the one covering the motor and the fuel injectors, not the plastic one at the front of the car) and look for loose lines, hoses or wiring.

Also - if you remove the shroud under the engine (you just have to pull out a bunch of plastic retaining plugs) you can get at the lower elements of the SAS. Look for loose lines, hoses or wiring.

You could be fortunate and just have a loose line. Does the caddie run OK?