AutoTap Express DIY logo
Tech Library
Introducing the All New AutoTap Express DIY

Your Vehicle Failed an Emissions Test

Many states now use a simple Onboard Diagnostics (OBDII) plug-in check instead of a lengthy tailpipe emissions test to verify emissions compliance. The OBDII tests are only used on 1996 and newer vehicles, and may be used in combination with a separate tailpipe test in certain situations (the rules vary from state to state).
  • Failed Emission
Probable Causes:
  • Eleven monitors
  • Catalyst Monitor
  • Misfire
  • Evaporative System
  • EGR System
  • Fuel System
  • Heated Catalyst
  • Secondary Air System
  • Comprehensive Component
  • O2 Sensor
  • O2 Sensor Heater
  • A/C System Refrigerant

The OBDII onboard diagnostic system that is used on all 1996 and newer passenger cars and light trucks (as well as a few 1994 and 1995 models) will set a fault code and turn on the MIL lamp if it detects ANY problem that MIGHT cause emissions to exceed federal limits by 1.5 times. Notice, we said MIGHT cause emissions to exceed limits. The actual point at which a code is set is determined by the vehicle manufacturer based on extensive testing and how conservative (or liberal) they are with respect to the rules. So, in many instances, the MIL lamp may be on even if the vehicle is not really creating a menace to the environment. In fact, some vehicles with a MIL lamp on will easily pass a tailpipe emissions test. Each state makes their own testing rules, and the rules have to conform to what the U.S. Environmental Protection Agency requires.

To pass a plug-in OBDII emissions test, a vehicle must:
  • Have a functional Check Engine Light and diagnostic connector (no tampering or funny business allowed).
  • The Check Engine Light must be off.
  • Successfully complete the OBDII system monitors that are built-in to the vehicle.

The OBDII system will run a number of self-checks (called “monitors”) to check the health of its engine management system and emission controls. Some of these tests run every time the engine is started and driven, but others (the catalyst and EVAP monitors) only run under certain conditions. Getting the catalyst monitor to run may require driving the vehicle for a number of miles under various speeds and loads. The EVAP monitor won’t run unless the vehicle has sat overnight and the fuel tank is between 1/4 and 3/4 full. It also may not run in extremely hot or extremely cold weather. AutoTap Express DIY will show you the status of the OBDII monitors. Figure 5 shows a complete emissions test status for a failed vehicle.

Figure 5

Figure 5

On OBDII vehicles before model year 2000, the rules may allow one monitor to be incomplete and still be accepted for testing. Some vehicles from 1996 to 1999 had ”monitor issues” that essentially mean some monitors NEVER run or set. Special allowances are made for these vehicles, or they may have to take an alternate test. If you failed the emissions test, you probably had a Check Engine Light on and one or more DTCs in your computer. Clearing the codes or resetting the OBDII monitors just before a test won’t help you sneak through because the catalyst and EVAP monitors need time to run, which also gives the Check Engine Light and codes time to reoccur. You have to diagnose and repair the fault before the vehicle will pass. If your vehicle was rejected for testing, it means all of the required OBDII monitors had not completed their self-tests. Drive the vehicle for a few days around town and on the highway, and try again. You can easily use your AutoTap Express DIY to check the status of the OBDII monitors before you bring your vehicle to the test facility.

If your vehicle failed a tailpipe test and the Check Engine Light is NOT on, chances are you have a problem with the OBDII system, a burned out MIL lamp, or a faulty catalytic converter. The converter is essentially an afterburner that cleans up the exhaust after it exits the engine. The OBDII system uses a “downstream” oxygen sensor to monitor the efficiency of the converter, and it should detect a drop in converter efficiency if the converter has been contaminated or is failing (ignition misfiring, leaky exhaust valves, and oil burning can all damage the converter). What you want to look for: Any conditions that might cause ignition misfire, an overly rich or lean fuel condition, or loss of compression. Use your AutoTap Express DIY to look at the oxygen sensor outputs, coolant temperature, airflow, calculated engine load, and inlet air temperature.