How Not to Get MIL-Stoned

Motor,  Apr 2004  by Bell, Sam

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Advanced modes. If you have access to menu items like "last test," "last test failed," "last test passed" or "tests failed this ignition cycle," be sure to use them. Knowing the last test that failed, along with the exact failure readings, allows you to zero in on the particular failure. Just as importantly, accessing a list of tests passed allows you to be confident that the problem won't return to haunt you.

Two-trip codes. Some codes will not set until the PCM registers a second failure of the same test. This can leave you in the embarrassing position of returning the vehicle with a DTC already pending. Always be sure to check pending codes (often a generic scan menu function, rather than an enhanced interface option) before you return the car.

Remember, too, that most state OBD II tests allow no more than two incomplete monitors for model years 1996 to 2000, and only one for subsequent years. If your state requires that the catalyst monitor must show Complete on retest, you may need to drive the vehicle until all criteria are met before returning it if the customer's test or retest date is imminent.

Returning the Car

Before returning a customer's car after extinguishing the MIL, sit down with him and go over the OBD II Freeze Frame and DTC Checklist. Take a few minutes to review the OBD II self-test features and explain the types of driving that will be required to run all monitors to completion. Here I usually return to my ladder analogy and explain that we have now completed only the first step. Then I enlist my customer as an ally in the battle against the MIL, getting him to promise to help me keep the vehicle running its best by letting me know immediately if the light comes back on.

OBD II regulations require that any detected failure that might result in emissions of a regulated tailpipe gas exceeding new-car standards by 50% or more will cause the PCM to command the MIL to light. Does this mean that emissions levels will be elevated every time the MIL is illuminated? Not necessarily. In fact, research suggests that most problems that illuminate the MIL are not accompanied by an appreciable increase in emissions, as measured by the Federal Test Procedure (FTP) used for new-car certification. An illuminated MIL does not guarantee that an emissions problem exists; it merely signals the likelihood of such a symptom. Additionally, some intermittent problems that cause the MIL to illuminate may no longer be present, even though the lamp is still lit.

New Tests

Many states are now moving to a new type of emissions enforcement program, popularly known as "plug and play." These new tests no longer measure actual tailpipe emissions for OBD II vehicles. Instead, typical requirements include:

*the MIL must illuminate at KOEO (bull) check);

*the MIL must extinguish at KOER;

*the MIL command state must be off (prevents tampering, such as wiring the MIL in parallel with the oil pressure warning light);

*no more than two (1996-2000) or one (2001 and up) self-test monitors shall report as incomplete. This prevents a freshly cleared car from passing until most monitors have run.