This comes from a reader named Joseph who is having trouble reading his OBD-II codes:

I purchased a tester (16-pin Data Link Connector for 1996 and newer cars) and so far used it twice-both times I got no code reads, it displays "0" -- I assume everything is ok. However, I hear a noise when the car reaches 40 mph though the car runs smooth as it's speed increases. I had a new muffler and pipes installed over a year ago-the car has been running fine. Please advise-what can I do?
Sincerely, Joseph

OBD-II Code readers are uniform for the most part, but there are subtle differences from brand to brand. Most of the readers I've seen will say "NO CODES" on the screen rather than just a "0." It is possible that for your reader the "0" means no codes were found. It could also be indicating an error. Check your code reader's manual to be sure.

If you are doing everything right and getting nothing but oddball or no info from the reader, you should troubleshoot the OBD-II brain to see if it's not reading correctly.