[Syty] Passed emissions test, but why?

[Peter]

Why does thermostat temperature have so large an influence on passing emissions?
   
  Twice now my Ty has failed emissions testing here in St. Louis, just barely failing HC, NOx, and CO.  The third attempt followed swapping the 160F thermostat for a 195F thermostat.  It "quick passed" with emissions at about 10% of the allowable limit.
   
  Exact same story as the last test two years ago.
   
  I presume the catalytic converter was not firing up with the 160F and came alive with the 195.  So why does the thermostat influence exhaust gas temperature so much?
   
  Or is there another emissions control mechanism at work?
   
  Best regards,
   
  Peter
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