I had an intermittent P1870 then yesterday the vehicle (99 Blazer) lost a LOT of power while driving uphill (SES was flashing) but now everything seems to be working fine although SES remains on. Codes P1133, P1870, P0131, P0132, P0135, P0300. Where do I start. Any suggestions as to what needs to be done. Should I start by replacing all o2 sensors?

 

First - clear codes, verify they are cleared, drive until SES light comes on, and report what codes come back.

 

most of your codes are for the o2 sensors bank1 p0300 is the fun one plugs wires, distributer and maybe rotor , if you haven't done these in a while you should, and be sure to use a/c delco for the wires plugs and distributor as the 4.3 dos not like other brands

 

I decided to replace the distributor cap and rotor button since I heard our vehicles don't like anything but AC Delco parts even though these parts were changed a year ago with some other brand. There was still no change, the miss was bad until it warmed up then seemed to run okay. This was repetitive with every cold start. I then ordered and installed all three O2 sensors and that seemed to fix my problem. The "Service Engine Soon" light went away after a few short drives and all seems well. Thanks for the help guys.