I have a 99 blazer 4.3 4wd auto. The P0135 light came on and replaced the drivers o2 sensor and cleared the code. 2 days later Same code comes back on. So I changed the passengers side with the drivers to see if it was a faulty o2 sensor. The same code appeared again. I checked the ground and the harness seems to be good. Checked the fuses. I'm thinking maybe the temp sensor might be out. In the picture it shows all the settings if that helps.

If you have any suggestions would like to hear them.

Hope someone can help this light is driving me insane

 

I am not sure if you have seen our DTC section, but here is a link to your specific code with the troubleshooting steps:
P0135 - HO2S Heater Circuit Bank 1 Sensor 1

I would ohm out the wiring to make sure that you do not have a problem with the heater circuit wiring.

How do the long term fuel trims look for B1 & B2?