I recall a problem with some Honda OBD-II vehicles where they would not turn on the check engine light when they were supposed to.
I believe this resulted in a recall, and a 150,000 mile
extension of the warranty on the O2 sensors.
Fact is, code is only as good as the ethics and competence of the person who is writing it. (Which of the two was lacking in the Honda case?). EPA regulations, in fact any laws, are only as good as the ability and willingness to enforce it.
As far as domestic OBD goes, both GM and Ford had quite sophisticated systems. GM had a data stream output long before anyone knew what OBD-II is, and Ford's EEC-IV had extensive self-test diagnostics. As one example,, compare the number of possible codes for a typical 1987 Ford EEC-IV system to the number of possible codes for a typical 1987 import system. Then remember there are three types of EEC-IV codes--memory, key on engine running, and key on engine off and that a single code could mean three different things depending upon what type of code it is.
As a final note, I will point out that long-term fuel trim on OBD-II is allowed to go as much as 25% positive or 25% negative before the light comes on and an error code is stored. I think you ought to start being concerned about the problem at 10%, and in fact I became aware of the presence of an otherwise asymptomatic vacuum leak when I looked at the long-term fuel trim on my old car--it was at about 22% correction, as I recall.
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Remember, lambda sensors are not installed to make the engine run "well", they exist solely to serve the catalyst. It's their only purpose in life.
If it were not for the oxygen sensors, that vacuum leak I described above WOULD have made the engine run like #@$%!. (It has a MAF sensor, not a MAP sensor).
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Even in one of my older cars (a 1987 import OBD-I system) there isn't a situation in which the sensor isn't monitored for every form of signal fault that would result in emissions exceeding 1.5 times the federal limit
So how much extra fuel is a car using when it is spewing out 1.45 times the federal limit in emissions compared to a car that's only spewing out the federal limit, anyway? (And what's that doing to the cats?)