On Sat, 19 Sep 2015 04:45:38 +0000 (UTC), Ewald Böhm
wrote:

On Fri, 18 Sep 2015 22:45:53 -0400, Ed Pawlowski wrote:

I assume the car's computer
knows an instrument is plugged in so it changes the program.

Very few states use OBD emissions testing, and certainly California
doesn't yet, where California is fining VW along with the EPA.

http://www.arb.ca.gov/msprog/smogche...bd_only_im.pdf

Most use tailpipe testing.

Some, like California, run the car through the Federal Test Procedure
on a dynomometer.

Given thats at least three different procedures (where each state can
easily be different), I don't see *how* the engine computer *knows* it's
being tested for emissions.

Since almost no states use the OBD method, that's why I asked how the car
knows it is being tested.
Don't know how other jurisdictions do it, but in Ontario the old
"drive clean" test was a "rolling road" sniffer test at two speeds,
with the car connected to the computer via the diagnostic port, but
not accessing discrete codes.

The new system does away with both the "rolling road" and the sniffer,
meaning it can only "guess" or "deduce" if the NOX is within range -
it cannot tell if the reduction catalyst is working because only the
oxidizing catalyst is monitored by the secondary O2 sensor.

It is POSSIBLE that VW implements the "over-ride" whenever a certain
sequence of events is performed that are substantially the same as the
initialization procedure for running the test (There is a perscribed
sequence of events that MUST be performed to get a valid test result)
(like 20 many seconds at a particular RPM, followed by another given
period of time at another RPM) which, if performed during the normal
process of driving would also put the system in "bypass" for the
anticipated duration of the test.