Q. How serious is it to drive with your check engine light on? My car starts and runs perfectly, so it can’t be something very serious? Everyone I’ve asked tells me something different.

Jessica H.

A. Jessica, your MIL (malfunction indicator lamp) illuminates to advise you that your OBD-II (on-board diagnostics) system has identified a fault or condition that will likely increase emissions 1½ times beyond the federal test procedure standard.

Powertrain diagnostic trouble codes (DTCs are a five digit code beginning with a P, such as P0440) are the only vehicle faults that will cause the check engine or service engine soon lamp to illuminate. Body, chassis, and network codes will likely go unnoticed unless a scan tool is used to check for them. The term powertrain includes engine, fuel, ignition, emission control systems and transmission.

Depending on the vehicle, there can be a sizable portion of the 1,688 listed powertrain diagnostic trouble codes possible, so taking a guess for the reason without retrieving DTCs is quite a long shot!

Here are the five most common check engine light causes: failed oxygen sensor (about 8% of all issues, average cost to repair: $260), catalytic converter degraded/failed (about 7% of all issues, average cost to repair: $1,200), faulty spark plugs and ignition coils (about 6% of all issues, average cost to repair: $400), loose or faulty gas cap (about 4% of all issues, average cost to repair: $20) and faulty mass airflow sensor (about 4% of all issues, average cost to repair: $380). Other causes trailing downward in percentage are typically faulty sensors, solenoids, wiring connection faults, dirty fuel injectors, leaky manifold gaskets, et cetera.

So, is it OK to keep driving with an illuminated MIL? No! As simple as it is to determine which code or codes may be present, it just isn’t worth the risk of possibly damaging one’s engine, transmission or catalytic converter. If an engine starts and runs well and the transmission shifts normally, the odds of a damaging condition are quite low, but is the average driver really tuned in closely to normal operation? I can think of one case where a misfire developed and wasn’t noticed during a long highway trip (larger engine/many cylinders, began with a slight thumping/roughness). The continued misfire torched the catalytic converter, resulting in a severe exhaust restriction, which led to engine overheating and massive failure. That was a $10,000 mistake!

OBD-II generic code readers are now so inexpensive (Autel MS309, $25, is a good choice) and easy to use, it makes complete sense to find a friend that has one, stop by an auto parts store for a free check, or purchase one in order to find out the facts. An online list such as obd-codes.com can be used to determine the general cause and seriousness of the issue. Keep in mind that throwing a part at a problem without following the DTC diagnostic chart is pound foolish. This might be OK in the case of a failed oxygen or air-fuel sensor heater (a very high probability it’s the sensor’s heater that’s failed), but it's a no-no when it comes to a fuel trim code, for example.

A final note, a flashing MIL is big trouble that should be fixed right away. This indicates a catalyst threatening misfire is occurring!

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at bradbergholdt@gmail.com; he cannot make personal replies.

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