Trouble Codes Anybody?

I have a 1998 Blazer 4.3 Vortec CSFI (4WD). Replaced many sensors with factory parts. Last one was the MAP Sensor. Now SES light is on. Codes are P0108, P0106, P0101, P0121 and pending P0106. All sensors are new, recently tuned up all GM parts. Fuel pressure reads 55 psi and holds. Had starting issue, car sat over the weekend and would not start up. It has spark at coil and fuel pump was working. The next day, got a pressure tester, tested it with key on, read 55 psi, hit the key and it started. It was 11 degrees outside. I also have a 1999 suburban with a 5.7 Vortec. Same CSFI. Starts and runs fine. No SES light. Fuel pump whines very loud. Used the pressure tester on it, it read 50 psi. I know the specs are 60 to 66 psi, why does the Suburban always start with less pressure than the Blazer, and the Blazer has starting issues? One more thing, I haven't changed the O2 sensors in the Blazer, but Toubleshooter checks out fine. Please reply.

Codes do not say to replace senszors but to check them out.

Did you clear codes and recheck for them to come back?
Or what code comes back first?

Also you need to get the fuel pressure up to par.
Low fuel pressure can cause other codes.

Give us the full complete fuel pressure readings.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

I think your gauge is inaccurate. My experience is that they won't start at 50 psi.