Your MX-5/Roadster and the MOT test exhaust emission test

Technical Area General Information
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Vehicles presented for test that were first used between <?xml:namespace prefix = st1 ns = “urn:schemas-microsoft-com:office:smarttags” /><st1:date Year=“1986” Day=“1” Month=“8”>August 1st 1986</st1:date> and <st1:date Year=“1992” Day=“31” Month=“7”>July 31st 1992</st1:date> will be subjected to a Non Cat. Test. This test applies the limits of 3.5% Carbon Monoxide (CO) concentration and 1200 parts per million (ppm) hydrocarbons (HC).

<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />  Any MX-5/Roadster with a fully functioning, well maintained engine management system with the engine at normal operating temperature will be able to meet these limits even with a decat pipe fitted.   Any vehicle presented for test that was first used on or after 1st August 1992 will be initially given a Basic Emission Test (BET). The emission limits set down for this test are a Carbon Monoxide concentration of 0.2%, a maximum Hydrocarbon concentration of less than 200ppm and Lambda must be between 0.97 and 1.03 at a fast idle speed of  between 2500 – 3000rpm (fast idle). Then at a natural idle speed of between 450 and 1500rpm (natural idle) the CO% must be 0.2% or less.   Any MX-5/Roadster with a fully functioning, well maintained engine management system with the engine and catalyst at normal operating temperature will be able to meet these limits.   If for one reason or another, your vehicle fails the BET, then the tester will follow a number of set procedures.   
  1. If your car was first used between August 1st 1992 and July 31st 1995, then the tester must look for an exact match in the Service Emissions Book. This book lists every model of vehicle and states the emission limits for each vehicle. In the case of MX-5’s the limits for vehicles of this age is 0.3% CO & 200ppm HC at fast idle and 0.5% CO at natural idle.
  By design, an MX-5 of this age with its catalyst removed will not pass this test  
  1. If an exact match can’t be found, as is the case for Eunos Roadsters as these are not listed in the Service Emissions Book, then the tester re-tests the vehicle applying the limits of CO at 3.5% or less and HC at 1200ppm or less at natural idle, a non cat test.
  A Roadster with a fully functioning, well maintained engine management system with the engine at normal operating temperature will be able to meet these limits even with a decat pipe fitted.  
  1. If your car was first used between August 1st 1995 & August 31st 2002 then again the tester must try and find an exact match in the Service Emissions Book. MX-5’s are listed and must meet the emission limits of less than 0.3% CO and under 200ppm HC at fast idle and 0.5% or less CO concentration at natural idle. Roadsters of this age, not being listed in the book are now subjected to a CAT test using default limits of 0.3% CO or less, HC 200ppm or less and a lambda reading of 0.97 to 1.03 at fast idle and a CO reading of 0.5% or less at natural idle.
  By design, an MX-5 or Roadster of this age with its catalyst removed, will not pass this test.   All vehicles first used on or after September 1st 2002 are tested to vehicle specific limits if an exact match can be found. A CAT test using default limits is carried out where an exact match can not be found.   This information was correct as of November 24th 2008. MOT regulations are constantly changing so it’s accuracy can’t be guaranteed for any period of time. The emissions levels change for different vehicles this info is specifically for Mazda MX-5 or Roadster and Eunos Roadster. Errors and omissions expected and accepted but the info is correct to the best of my knowledge.