New Parts Can Be Defective

February 14, 2012

I recently performed some repairs on my 1994 GMC Sonoma pickup. Along with replacing the fuel injector due to a leak from the fuel pressure regulator, I also replaced the spark plugs, air filter, and fuel filter.

Not long after these repairs were completed, I began to notice a slight engine misfire when pulling away from a stop. Over the next few weeks, the misfire became progressively worse. What could have gone wrong?

I suspected a spark plug wire was failing. Even though I was careful when removing the plug wires to access the spark plugs, no mater how careful you are, a wire can still be damaged.

I did not want to replace the spark plug wires without first knowing for sure they were the problem. I finally had a chance to diagnose my problem over this last weekend. I checked and rechecked the spark plug wires and everything was okay. When I was finally able to isolate which cylinder was causing the problem, I removed the spark plug (which I had installed just a few weeks ago) to have a look. As expected, it looked fine.

To rule out the spark plug as the culprit, I switched it with the one in the adjacent cylinder. Sure enough, the misfire moved to the adjacent cylinder. This “new” spark plug was defective, right out of the box! I was able to replace the defective spark plug and I am glad to report that my truck in running fine and is ready for its smog inspection.

There is nothing that breaks down communication faster between a technician and a vehicle owner than a defective new part. The owner knows their vehicle is not fixed correctly and can understandably be quick to blame the repair shop for a faulty repair. The technician is equally frustrated because a good repair has gone bad for some unknown reason.

My failed spark plug was fairly easy to diagnose; however, condemning a brand new part as defective can sometimes be very difficult. The technician knows the part is new and rightly assumes that it should work. Often times, things must be double and triple checked before the final determination is made the new part is indeed defective.

When a repair goes bad, this is the time when trust between a customer and their mechanic is pushed to the limit. The best thing for everyone involved is to take a deep breath, address the situation calmly and logically, and wait for the re-diagnoses and repair to be complete before placing blame. There are a number of different reasons a repair may go bad—misdiagnoses, poor workmanship, etc. But you never really think that a part can be bad until you rule everything else out.

Please share a story you may have of a repair gone bad due to a defective part.

About the Author

Jim Taddei has been in the automotive field since 1975 and has over 25 years of experience with General Motors products, achieving the designation of GM Master Technician. He is also currently certified as an ASE Master Technician, and holds an Advanced California Smog Check License. He has been the lead technician and team leader at a multi-line dealership. After leaving the dealership he spent a couple of years working in an independent shop and now uses his experience and expertise to help verify the quality of RepairPal Certified shops.

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