Under the Hood: Do not ignore the check engine light

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

A: 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 1 { 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, any of a sizable portion of the 1688 listed powertrain diagnostic trouble codes could be the culprit, so taking a guess 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: $1200)

• 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)

• Faulty mass airflow sensor (about 4% of all issues, average cost to repair: $380)

Other common causes are faulty sensors, solenoids, wiring connection faults, dirty fuel injectors or leaky manifold gaskets.

So, is it OK to keep driving with an illuminated MIL? No! As simple as it is to determine which code(s) may be present, it just isn’t worth the risk of possibly damaging the 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 oops!

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 get one. An online list such as www.obdcodes.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.

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