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Discussion Starter · #1 ·
Hey guys. I have a 2005 Frontier with 175k. I drive roughly 1000 miles a week and about 200 a day. I have owned the truck since 115k and figured I would do some good maintenance to it since I bought it used. About a month ago I decided to replace the spark plugs while I changed my oil. All went smooth until I went through the first tank of gas. My engine light popped up with a high idle code, and both primary cats (420,430). I took it to a shop to get the high idle relearned (figuring that would solve my gas mileage issue and worry about the cats later) and it went smooth. The truck now idles fine with no high idle codes. But my P0420 and P0430 codes are still showing and I still have god awful mileage. I called my local Nissan dealership and they said the cats either went bad, a sensor went bad, or my spark plugs were not the correct ones. I pulled my plugs and they are what I bought, NGK platinum tip plugs with the proper gap.

I really and truly have a hard time believing BOTH of my cats went at the same exact time and they are both bad. I beat the heck out of them and they feel solid. The exhaust doesn't smell bad. I was averaging almost 21 MPG(all 200 miles a day are highway at 70 MPH) and now I'm down to 17.3. And yes, I do calculate it off of gallons per mileage not just the fuel gauge.

If ANYONE has any insight on what the issue may be please put your .02$ in. I have a feeling its a sensor somewhere or possibly the MAF when it was removed for spark plugs. I have no idea. Any suggestions??

Thanks
-JP
 

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P0420 Catalyst system efficiency below threshold bank 1
P0430 Catalyst system efficiency below threshold bank 2

Cause of this failure includes exhaust leakage in exhaust pipe, or near exhaust manifold. Invalid catalytic box, error in ECT sensor, engine misfire or timing retarded. Too dirty engine oil, O2 sensor error, or too high fuel system pressure.

Both cats bad at the same time, weirder things have happened. But like you said it's weird it happened right after you did the oil and spark plug change.

Good luck, let us know how it all works out.
 

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I am going with some kind of leak in the intake track that you removed. Go through and make sure all gaskets are good, and intake track fitting and hose clamps are tight.
 

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Could also be 02 sensors. Do you have a scan computer with live data logging? You could look at your bank 1 and bank short term and long term fuel enrichment. Data logging can be helpful so you are not throwing parts at the truck.
 

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Discussion Starter · #5 ·
I have checked and rechecked all the intake hoses. I went through and checked everything I had touched while doing the spark plugs. I do not have a scan computer. Might be worth looking into with all of this going on. It very well could be an 02 sensor. I really don't understand what all is going on. I really hope both cats haven't gone bad that's a pricy fix! Thanks for all of yall's input so far. Keep it coming!
 

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Gosh, I wish I got 17 mpg on the highway. I get about 15-16 highway driving 75-90mph.

Does seem odd that the cat's would both go bad. Do you have a loss in felt power or anything?
 

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So has anyone figured this out? I have the same exact issue 10~12MPG down from 17~21. Had the Cat codes changed them all out last fall and just recently put new (more expensive) 02 sensors in. Still have the P0420 and P0430 codes. No vac leaks that I can find, no loss of power, but horrible gas mileage. I have FIXD plugged in and it shows voltage for oxygen sensor 2 @ .16 Volts, oxygen sensor 6 (not sure where this one is at) @ .25v @ 2200 RPM. Was told from my mechanic that the computer needs flashed from a Nissan Dealer. Nissan said they only flash the computer for transmission replacements, and at $125.00 and hour they would be glad to reset the CEL! then do a diagnostics at $125 an hour. Will post some live data later from the FIXD code reader along with some temps before and after Cats. 2008 Nissan Frontier SE v6 Crew Cab - 201,000 miles..
 

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You're close to needing your 3rd set of spark plugs. Should you swap in some new ones you'll get a good luck at the old ones' appearance. This may help determine if you have any injector issues.

Also, you said all cats were replaced...including both secondary cats? Any chance your muffler is plugged from broken-off cat material or a rodent nest in the exhaust system?
 

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I replaced all 4 cats and 2 air/fuel sensor with ntk, back 02 sensors were some off-brand but had great reviews. I actually changed out plugs about 1.5 years ago when I bought the truck. Not knowing what the previous owner did or did not do, i just changed them out to NGK plugs. Going to try to reset the computer module this evening (unhook battery) for about 20 minutes, then drive it tomorrow and see what happens.
 
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