A while back I saw a thread with a link to the BAT Auto Technical Code listings. It is a good article in terms of how to obtain the fault codes using the various interfaces on the car.

My particular car is a 1990 300 SEL. I have been successful using some of the more obvious codes to troubleshoot problems such as a bad EGR valve which corresponded to a specific fault code (16).

My question is how do you corellate the more obscure codes or codes that appear in combination to actual fault procedures? Is there a cross reference that has this information or do you have to develop your own approach based on automotive systems knowledge?

Just today I got two faults codes together: F/C 11, Air Injection System; and F/C 17, O2 Sensor Signal. I'm not sure where to begin troubleshooting. I know I can check the O2 sensor duty cycle on the test connector X11/4 on the left fender but how does that relate to the Air injection?

Thanks for the help,
Motorhead

<< Is there a cross reference that has this information or do you have to develop your own approach based on automotive systems knowledge?
>>

There are code remedy guidance charts , but system knowledge is the key.. [and system inter-relation]
You have a good example right here....
02 sensor code coupled with AIR code could be an aging 02 sensor that does not read well at lean conditions.
So, why would an AIR/02 sens code point in this direction??
B/C the computer does an AIR operation test by looking at the 02 signal when the AIR is energized... if the sensor does not report back to ECU that it has seen a very lean condition [ low volt ] at this test. it pops an AIR code...[ the AIR system can be fine , but the ECU/DM has not recieved an OK verification sig from 02].
Knowing this , one would do some 02 sens testing, specially
the 02 lean capacity....
Sytem knowledge and some common sense on the inter-relationship of parts/systems...a couple of codes ... some diagnostic equipment to test your conclusion... and you are there.
Many un-needed parts are changed due to lack of knowledge b/c a code charts says 'xyz part system' is misread as a bad part...

A code will tell you what street you are looking for , but not what house....

Outstanding description Arthur,
with the newly implemented OBD testing here in St louis for emissions, were are struggling to train the 'techs' to look at overall system relationships rather than replacing parts based on fault codes only.

Wish we had you as a trainer.