My 2000 Blazer's SES light came on about a month ago. The garage said the code required the (front) O2 sensor to be replaced. That worked for 2 days and came back on. I Took it back to the Original garage, but their computer was broken. I went to Autozone and took the printouts to the garage. It said random engine misfire. They now replaced the Wires,plugs, and distributor. Gas mileage dropped 25% to 8mpg. No Power, 5000 rpms equal 20 mph. Took it back and they replaced the rear O2 sensor. Everything back to normal. It ran great for 3 days. Today SES light came back on. Very frustrating and expensive. Any Ideas? Thanks.

Codes do not say replace sensors.

Post back the code no it had and now has.

How many miles on it and what engine?
Any test done on fuel pressure or cat back pressure?

What brand of spark plugs?
Fuel and air filter checked or replaced?
Can you feel a engine miss?
Still have missfire codes?

I would run a good fuel pressure test on it.
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 and fuel pressure testing guide line.
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.

Thanks, MT-2500 for the fast response.
As I mentioned, the car had no power. It hesitated like crazy. To go up a hill it ran about 5k to 6k rpms for 15 to 20 mph. This stopped after second sensor was replaced. The fuel pump and pressure was tested, but I do not know the numbers. This was done before the 2nd O2 sensor was installed. They said everything was in the normal ranges. Fuel pump was replaced in 11/2007.
The plugs installed are AC Delco.

The current error code says:
PO152 HO2S-21 Circuit condition.(Heated Oxygen Sensor Bank 2 Sensor 1)
Probable cause
1 Oxygen sensor defective
2 Fuel system running rich or lean
3 Engine misfire condition. (not misfiring now)
4 Fuel Pressure Problem (Fuel Pump or Regulator)
Is it possible the sensor went bad?
I put some Lucas Fuel Injector cleaner in the tank today.
Thanks, John

Is it now running good outside of check engine light or not running good still?
Could be sensor bad or bad wiring.
But also could be rich running /missfiring engine.
Bad Tps sensor will cause it
Make sure eng is is no running rich or misfiring.
I would run fuel pressure test first.
Let us know how it goes.
Good luck