[trooplewis]

My Nissans (the 3.0 and 3.5 V6's) were famous for sometimes throwing a code that didn't actually cause the CEL to come on. You would only see them if you happened to hook up a reader to the OBDII plug. The most common hidden code was when the Knock Sensor went bad, which greatly affected performance.

Ever have that happen with the V6 Mustang??? Do ours throw hidden codes?

 

[97GT12]

OBD has pending codes. The MIL won't be commanded to light until the problem repeats a number of times. This way an errant misfire or other transient condition doesn't create alarm.

Now there may be other manufacturer based codes that don't require lighting the MIL, but I don't know what they would be off hand. I suppose I could go digging through the emissions and diag manual I finally got a few weeks ago.