Under the Hood: Have patience before ditching Golf

BIZ_AUTO-VOLKSWAGENGOLF_MCT
The 2015 Volkswagen Golf family

Q: My daughter has a 2002 Volkswagen Golf facing a smog check by next month. It didn't pass because its catalytic converter apparently needs more mileage to register and store information from the monitors. I understand that, and we can drive it more to accomplish that goal. But we did just get that information cleared to turn off the check engine light. The underlying problem may be the ignition coil. Over the last few years, it's been replaced three times. I suspect it has caused a problem with the converter, and even if we swap out the converter ($900), we may have to replace the coil again. Does this car have an inherent weakness in the coil? Is there a chance we can get by without replacing the converter? Should we donate the car and replace it? What would you do?

Thanks for your advice,

–Jim

A: It sounds like someone recently cleared trouble codes from the Golf's engine control module, or the battery may have been disconnected. Either of these actions clears the emission monitors, which contain on-board diagnostic test results needed for emissions certification. Some monitors are fussier than others and it appears your catalyst efficiency monitor is being a straggler to finish its business. Assuming the oxygen sensor monitor has run to successful completion and no other faults are present, the only thing holding back the catalytic converter monitor should be the right combination of driving conditions. These vary by vehicle. I'd try, after a cold start, some city driving followed by a steady highway cruise for perhaps 10-15 minutes. Monitors will eventually run to completion whether the tested component/system is good or bad. If bad, a diagnostic trouble code will indicate the general fault area.

VWs of this vintage are notorious for ignition coil problems, and a misfiring engine can overheat and damage the catalytic converter quickly. A degraded/worn cat causes ineffective exhaust treatment, and an overheated cat can come apart internally, resulting in power-robbing exhaust restriction in addition to increased emissions. I researched a half dozen aftermarket ignition coil suppliers and can see multiple improvements over the original coil design, mitigating problems with internal arcing and insulation leaks. Assuming your Golf is equipped with the 2.0 liter four cylinder engine, the ignition coil unit actually consists of four coils – one for each cylinder – integrated into a single assembly. Failure symptoms of a coil of this type would probably be a single cylinder misfire, either occasionally or all the time. Engine roughness/thumping during acceleration would be the most likely noticeable effect, possibly accompanied by a constant or flashing (severe misfire) check engine light.

It's impossible to say if you'll need a new catalytic converter until the onboard diagnostic system finishes its cat test and a trouble code either appears or doesn't. An inexpensive OBD-II scan tool is great for this, listing monitor status as well as any DTC (diagnostic trouble codes) that may be present. Should a code indicate cat problems, further diagnosis is prudent prior to replacement, along with a careful check for misfiring issues. An available secondary voltage test can quickly prove out coil functionality. There are many aspects to making the decision to retire a vehicle. If the Golf is in nice shape and your daughter likes driving it, repairs of this type may be just an unpleasant bump in the road.

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ABOUT THE WRITER

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.

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