My 1991 s10 blazer 4x4 is so dang reliable but this last week I saw the service engine soon light flash and then go out. It did this off and on for a week until it stayed on long enough for me to make the codes flash. Code 13 was the culprit. Some sort of o2 sensor fault not a lean or rich indication. "o2 sensor voltage stays between .35 and .55 volts for 30 seconds while the tps voltage is above .55 volts. The engine has been running for at least 40 seconds and is at least 110F". So can anyone translate? Has anyone had this fault before? Should I go buy a new o2 sensor? I love this automobile and this is the first time the engine has ever acted up, except the water pump, alternator, belt tensioner pulley, and running out of gas a dozen times with a quarter tank indicated. But all of the failures have at least been spectacular to see.

 

More than likely your O2 is bad. When was the last time it was replaced? Be sure to use a quality sensor like an AC Delco, but stay away from Bosch.