If the Check Engine/Service Engine Soon light is illuminated on your dash, the computer of your vehicle is seeing an error that affects the performance or the emission operation of your vehicle.  We recommend that you do NOT take it to DEQ.  The vehicle will fail the test and it will be recorded in the vehicle history with DEQ what the failure is.

The vehicles’ Power Train Control Module (PCM) or Electronic Control Module (ECM) performs ongoing tests of the different systems of the vehicle while driving.  These tests monitor system readiness and completeness, if a system fails it will signal a diagnostic trouble code (DTC) and most of the time the vehicle check engine light will illuminate on the dash. The systems it tests are all emissions/performance related and are listed below:

  • Catalyst (CAT)
  • Heated Catalyst
  • Evaporative System (EVAP)
  • Secondary Air System
  • Oxygen Sensor (02)
  • Oxygen Sensor Heater
  • Exhaust Gas Recirculation (EGR) and/or
  • Variable Valve Timing (VVT)

How this applies to DEQ is that if your vehicle fails at DEQ, then the system it failed for must be repaired and it has to be driven enough (multiple drive cycles) to have that system reading ready and complete on the systems test results stored in the computer or it will not pass.  If the vehicle has not failed at DEQ, then you still need to repair that system in the vehicle to take care of the problem that caused the DTC so that it will not reset. But the system does not need to read complete or ready to pass. A gas vehicle from 1996-2000, and a diesel vehicle from 1997-2008 can pass the DEQ Emissions Test with 2 monitors reading unready or incomplete.  A gas 2001 & newer car, and a diesel 2009 & newer can pass with 1 monitor reading unready.

What this means as an example, if a repair is performed on an EVAP system to get the system monitor to read ready and complete it can take anywhere from 50 to 1000 miles of driving to get that test to run and complete.  You also need to make sure you keep the gas level between ¼ and ¾ of a tank or it will not run.  There has to be cold starts completed after up to 8 hours in between driving. Drive cycles can be lengthy and complicated.  We recommend if you have a Check Engine Light or Service Engine Soon Light have it diagnosed and repaired BEFORE going to DEQ for the emissions test.  Happy Motoring!


Karen Johnston

All About Automotive, Inc,

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