XPost: alt.home.repair, ca.driving   
   From: kludge@panix.com   
      
   Arlen   Holder   wrote:   
   >The goal is to learn how to diagnose a P0421 using an OBDII scanner.   
      
   Okay.  You're getting an error which is saying that the ratio between the   
   forward O2 sensor and the rear O2 sensor is not what it should be.   
      
   And you are -not- getting an error that is saying that the forward O2   
   sensor is out of range, which is also important.  The fact that you are   
   -not- getting that error tells you that the exhaust coming out of the   
   engine isn't THAT far off.   
      
   Now, it could still be off.  It's still worth doing all the basic sanity   
   checks: change the plugs, make sure the cables are good and there are no   
   signs of arcing, make sure fuel pressure is good and that the exhaust smells   
   okay.  The idle RPM should be right on the mark and should not be drifting.   
      
   If you're spitting unburnt fuel down the tailpipe, the converter is going   
   to be unhappy.   
      
   Now, look at the data for the two O2 sensors and look at what happens when   
   the engine starts up.  The first sensor should climb up into the normal range   
   once it warms up.  The second sensor isn't going to get into the normal   
   range until the converter warms up.  The shape of the time-domain curves   
   will be different from one model to another, but just watch.  The second O2   
   sensor might be bad... the converter might be bad..... and it's -possible-   
   that the converter is just overwhelmed with junk (although that it is least   
   likely of the three since you aren't seeing any other errors).  But watch   
   the numbers.   
      
   And you want to look at it for as long as you can... because you can have a   
   loose O2 sensor cable, or a bad engine ground, and see weird spikes on those   
   graphs that occur very very seldom but which can lead the computer to set   
   an error.   
      
   If this engine has two banks, you can compare the two and that is very very   
   useful information.  Since you don't give the engine or model it's hard to   
   tell if that will help you or not though.   
   --scott   
   --   
   "C'est un Nagra. C'est suisse, et tres, tres precis."   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)