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I am stumped to figure out this issue I had with my 2006 Hyundai Tucson with Automatic Transmission and i4 engine.

So I am experiencing some feel of power loss, however I got the car not too many months ago and had to do a transmission rebuild. So currently I'm not getting any codes but I noticed when I carry my family (wife, 2 infants) it lacks power to climb steep hills.

I did the digital laser temperature check on my first cat and the upstream was registering around 350 F and my downstream was about 425 F when running the engine at 2.5k rpm, than I scanned my second cat which is under the car and it was registering ~345-350 on both ends, like it wasn't having a catalyst reaction. Am I looking in to the wrong thing here, or do I have an issue with the 2nd cat?

The 2nd cat appears to be bolt on only but the bolts are completely rusted up otherwise it may have been an easy thing to take down and look through it.

The way my exhaust is designed it starts with a 4-2-1 exhaust manifold, and the 1st cat is welded onto it, than there's the L bend, a flex pipe, and a straight pipe to the 2nd cat, than it goes towards the muffler. My O2 sensors are located before and after the 1st cat only, the 2nd cat doesn't have any O2 sensors on it.

I'm experiencing about a 15% reduction in MPG as well.

On another note, I get this weird thud coming from under the car while I'm backing out of my drive way on a turn, where if one front wheel is on even pavement while the other wheel is on my sloped driveway there's this weird kabump sound. Maybe that's where I should be looking instead of at the cat?

Thank you all for any help you could give.

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Huh? A catalyst converter is only there to help burn unburned fuel. It has no effect on the power output of your car.... With that said, however, if one of your cylinders is not receiving a spark, that will send a whole bunch of unburned gasses to the cat and it will get mighty hot. Obviously, if you are not running on all cylinders, the car will have poor power.

We really want to know if there are any OnBoard Diagnostic (OBD2) codes generated by your engine control computer. The engine control computer controls air/fuel mix plus a whole lot more. For most low power issues, the system will set a code. (I said most, but not all...) There are some defects that may not generate a code...

  • Vacuum leak from old cracked hose in engine compartment
  • ignition troubles (e.g. a spark plug wire grounding out to the block)
  • plugged up air filter
  • dirty fuel filter
  • dirt/water in fuel tank
  • damaged fuel line, restricting flow
  • no air gap at A/C compressor (causing the A/C to run ALL the time)
  • etc...

The driveline thud thing is odd, but I doubt that is causing your power loss issue. You would definitely want to understand what is causing that though! I suspect stabilizer bar bushings or Constant Velocity (CV) joints or perhaps the differential gears inside the transmission as the likely source of that noise. (I'm assuming front wheel drive only?) You might also look at front shocks, and suspension springs and seats.

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