Friends daughter's 1996 Blazer,auto,4.3 is throwing this code w/ this explanation:ECM has detected the switch ratio of rich to lean and lean to rich was below a determined number. Probable causes being: 1. Failed HO2S-21 heater ckt. 2. HO2S-21 signal shorted the ECM ground. 3. Improper oxygen supply to sensor- check sensor connector and wires. Can someone enlighten me as to what this all means? Thanks. Dan

 

Another post from Philadelphia International Airport.

I'm guessing the code was P0154 (description matches).

P0154 Explained

That'll be the book description and diagnostic chart. The first thing I would do would be to swap the upstream sensors side to side. The sensors you want to swap are the ones closest to either side of the engine. The B2S1 sensor being on the passenger side and the B1S1 on the driver, both being ahead of the 'Y' in the y-pipe. If the code follows the sensor, then the sensor is bad.

I would recommend that you replace the sensor with a genuine AC Delco sensor. My testing with various brands of sensors, watching their new, just out of the box, activity has shown the AC Delco's to be supperior in GM applications.