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Spark plug replacement leads to more problems

Hello,
I have a 2014 Nissan Sentra. I started noticing that the car was driving sluggish, gas mileage was getting worse and just didn’t feel right. However, it was still getting power to the engine and could go on the highway fine. First thought was I needed a tuneup and new spark plugs. Took it in and they replaced the plugs and when they did the test drive afterwards they said it was bottoming out and they barely got it back to the shop. They want me to replace the catalytic converter. Now I know that bad plugs can lead to CC failure, but could they have messed something up while taking out the manifold and changing the plugs? Thanks.

Was your check engine light on at idle . . . or flashing?

Nope, not at all. I could just tell that it was driving right. Love the avatar by the way.

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Taking out the manifold???

How many miles does this 2014 Sentra have, and does it have a salvage/restored/rebuilt title? If it has less than 80,000 miles, and has a clean title, then the catalytic converter(s) should be covered under the federal emissions warranty. Therefore, you should not have anyone other than a Nissan dealer do this repair.

Also, you should be aware that several Nissan engines have a terrible design feature which uses overlapping intake/exhaust valve timing as a substitute for having an actual EGR valve and pipe connecting the intake and exhaust manifolds. On certain Nissan engines, there is a known problem where the “pre-cat” located in the exhaust manifold comes apart and the substrate gets sucked into the engine, ruining the piston rings and cylinder walls. This is a serious enough problem that aftermarket exhaust manifolds are available which contain no “pre-cat”, or which are equipped with a special screen which will prevent any particulate matter from being drawn back into the engine.

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Bill the cat

image

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Sounds like the spark plugs were not the cause of the performance problem that you were experiencing, the shop found an exhaust restriction.

Thanks everyone. They showed me the spark plugs, they were certainly bad. So there’s nothing that a mechanic could accidentally do to mess up the C.C.? Covering all my bases.

That’s what he said

A lot of other brands with variable valve timing have that design, without issues.
My 2006 Toyota Matrix and current 2017 Hyundai Tucson have it.
I don’t think it’s so “terrible”, not having to ever clean out clogged EGR valves or passages.

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Unless they physically damaged/crushed the catalytic converter, for example by placing a floor jack under it while trying to lift the vehicle, there is nothing the mechanic could have done to damage it. If the cat is bad, the mechanic who worked on your car did not cause the problem.

Thank you for the succinct answer. So the only way I could find that out is to have another repair shop check it out. I just don’t understand why my car was more or less fine to drive before I took it in and now I essentially have to floor it to get it to go.

This would be a great way to crush something in the exhaust system. Did you also have a problem with the car bottoming out or was the shop playing Dukes of Hazzard with your car?

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Never bottomed out. Just felt like I was losing some horsepower as a car always does when it has been a long time since the spark plugs were replaced. My check engine light wasn’t on or anything. I was being proactive.

There’s obviously a problem with terminology . . . and how it’s used . . .

When op says “car bottoming out” . . . he apparently doesn’t mean the car bottomed out onto the exhaust system

it sounds like he means the car was gutless and couldn’t get out of its own way . . .

At least that’s how I interpret it

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You are probably right.

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