Chevrolet Repair: engine code 13, throttle positioning sensor, oxygen sensor

Question
Scott,
    I have a 1992 s-10 blazer 4x4 with a 4.3L vortec v-6.  Recently my check engine light started coming on then going off after a few seconds.  I checked the code and it flashes a code 13.  My paperwork indicates this is an oxygen sensor or a throttle positioning sensor.  I first changed the O2 sensor and disconnected the battery to reset the computer.  The code still came on.  I then replaced the TPS and reset the computer again.  The check engine light is still coming on.  The truck does not seem to be running poorly.  Do you have any other ideas why the code 13 would be stored.

                                   Thanks, Brian

Answer
Hi Brian,

It is probably the O2 issue.  The reason is not that it is bad, but that it is not cycling.  You see, the computer needs to see the O2 reading continuously going up and down between 0 and 999.  If this is not happening, it assumes the O2 is plugged up and bad.  

Now, why won't it cycle.  You are running too rich or too lean.  0 is a lean, 999 is a rich.  You are probably too rich...just a guess.  This is probably caused by a leaky fuel injector.  Dripping gas into the cylinder and not completely burning off.  The O2 sees this as a rich condition and won't cycle.  It will instead stay around 0.  This is because it is trying to correct for the rich condition.  When the computer sees the car is running because of MAP sensor signal and ignition module signal, it figures the 0 is a dead O2.  

You can find the bad O2 by looking for a bad or fouled plug.  That plug has the bad injector.  

You could also just need a tune up.  Dirty air cleaner, worn plugs and wires can cause issues.  Dirty air means less air and leads to a rich condition.  Incomplete burn because of bad plugs is also considered rich condition.

C J S