Why is OBD2 so stupid...or slow!

[Stic-o]

It happen more then one time that I notice there is a problem, diagnose it in my head and figure out what needs to be fixed. This all happens probally in the first mile or two of the problem. Mean while 20 miles later OBD2 says..ohhh there's a problem, I should turn my check engine light on

Today on the way to my dad's I noticed the speedmeter stoped working...hmm must be the VSS. It the only logical thing that would just stop working... So I leave my dads to go home and deside to stop at pep boys on the way home and pick up a new sensor. On the way to pep boys, the check engine light come on. So I replace the sensor in the PB parking lot and continue home and now the speedometer works again. When I get home I check the engine light and it says there a speed sensor problem! DUHHH!! Stupid OBD2

 

[rwenzing]

It takes 2 successive trips with the same fault to turn on the CEL (to help minimize false codes).

After 3 successive trips without the fault, the PCM turns the CEL off (to ensure that the CEL will be extinguished after the problem has been corrected).

Then it takes 40 successive trips without the fault to erase the code from PCM memory (to keep the fault code available for diagnosis of intermittent problems).

 

[Spdrcer34]

wow....didn't know the method behind the madness.....now I do...

Thanks,
Ryan

 

[dogfriend]

In the case of the VSS, even though it hadn't set the code at first, you could have looked at the PID with a scan tool and picked the problem up that way.

A couple of years ago, I was having a misfire problem but wasn't getting any codes, but when I set my scanner to look for the "Misfire True", the scan tool would read "True" about 1/2 sec after I could feel the misfire. I never did get any codes, but the problem went away after I replaced the coil pack. The resistance was about 1k ohm higher than spec.