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Discussion Starter · #1 ·
Investigating a rattle noise under the front end as soon as the engine is started. Don't really notice it when driving. What I DO hear when driving is what sounds like (to me) to be an exhaust leak way in the back. Quite noticeable, droning when just cruising around town at lower RPMs.

One or more O2 sensors is also now not happy -- P0420 code. So now I'm investigating an exhaust leak before I worry about sensors. Otherwise it seems to run fine? 88K miles.

No smell inside while driving, maybe a whiff under the hood. Inconclusive, actually.

I haven't yet been able to raise the front to take a proper look under the engine. I was able to give a quick look underneath on the passenger side. I don't really hear anything coming from the front underneath while crouched on the ground next to the front passenger door, maybe it's a little louder in the midsection at the resonator. Walking around anywhere near the muffler = silence. I need to repeat this with someone goosing the throttle.

The rear cat is connected with exhaust clamps, not flanges. Is that normal on a 2006? I'm seeing some replacement parts online but with flanges. What seems really odd is that at the front of the rear cat there are curiously TWO exhaust clamps. And they look really new.

Tire Automotive tire Motor vehicle Automotive exterior Bumper

My understanding is that there's a total of 3 catalytic converters, with one each at each cyl bank built into a manifold collector and then the front Y-pipe connects them both to the rear cat, is that correct?

I wouldn't mind ripping it all out if i thought I had sufficient access to do so. I don't have a welder, but I know how to use a ratchet. Unfortunately, there are welded sections here.
 

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It is common for rattling to be caused by catalytic converter heat shield rusting loose. A lot of times this can be solved with a hose clamp.
For exhaust leaks, the flanges can leak if you have any engine mounts broken. Another common place is the flex joint.
 

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Discussion Starter · #3 ·
Yeah I suspect a heat shield of some kind up front. You can hear it on a cold start, drive it a short while and then the rattle noise from down there is "gone." I'm hoping to inspect the rest as soon as I can get under it this weekend.

Not entirely sure I'd know how to inspect a motor mount however. Presumably you'd "safely" raise the engine a little and look for separation, while hopefully not doing damage to that which was previously undamaged prior to the procedure (!)
 

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Discussion Starter · #4 ·
So ... from looking at pictures of the radiator side and firewall side manifold catalytic converters and they have a mounting boss for two, presumably so that each manifold catalytic converter performance can be measured independently.

Total of 4 sensors, and a particular code is only revealing one "set" that's the problem, in this case the on the firewall (bank 1) side. Could be the cat, or either of those two sensors on that cat.
 

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Yeah I suspect a heat shield of some kind up front. You can hear it on a cold start, drive it a short while and then the rattle noise from down there is "gone." I'm hoping to inspect the rest as soon as I can get under it this weekend.

Not entirely sure I'd know how to inspect a motor mount however. Presumably you'd "safely" raise the engine a little and look for separation, while hopefully not doing damage to that which was previously undamaged prior to the procedure (!)
If you can have someone hold the brake firm with their left foot, shift into drive and rev the engine while you observe, you look to see if the engine raises up out of the cradle, do the same in reverse to check other mount.
 

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Discussion Starter · #6 ·
... Total of 4 sensors, and a particular code is only revealing one "set" that's the problem, in this case the on the firewall (bank 1) side. Could be the cat, or either of those two sensors on that cat.
I think I was wrong? P0420 is weak converter performance on Bank 1. There are actually 7 different codes related to the Bank 1 O2 sensors. So far I'm not seeing how P0420 is anything but a bad catalytic converter on Bank 1, whether internally or broken and allowing exhaust to escape and cause a bad reading that way.
 

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I wouldn't say right away that cat replacement is neccessary. With some investigation on top of the engine and a little tweak here an there it can be reversed if the cat is not to far gone.
Is your 06 Pilot a 2wd or 4wd?
 

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That's good, you dont have to deal with VCM technology. 👍
Not knowing the history of your vehicle, I want to mention the necessity to know that the timing belt water pump job is up to date. With that out of the way, I'd just start simple...
  • Air filter New/Clean
  • Inspect Air intake hose for cracks. Replace if needed, should be airtight. If it's been this way a while, it may be neccessary to clean the throttle body inside and out.
  • Spark plugs New/Clean NGK Laser Iridiums
  • Spark plug tubes free of oil, if oil is present replace valve cover gaskets, grommets and spark plug tube seals. Replace the intake manifold gasket if doing this job.
  • Coils cleaned of engine oil, all firing
  • Replace PCV valve
  • Replace EGR valve
  • Clean EGR passages
  • Use full synthetic 5w20 oil, no Extended Performance oil or blends.
  • Use top tier 87 octane fuel or use injector cleaner regularly
This is my punch list to zap that emission code and keep other gremlins away.
 
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Discussion Starter · #10 ·
Always a good list, thanks. Several (EGR, cylinder misfires) would actually have their own codes if bad and would likely reveal themselves in driving. This one runs smooth and has no contributing codes at all that would lead the cat to become unable to correct.
 

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It is common for rattling to be caused by catalytic converter heat shield rusting loose. A lot of times this can be solved with a hose clamp.
And if so, watch how EricTheCarGuy does it.

 

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Some have successfully installed the Walker exhaust kit. If the exhaust leak is the reason for the emissions code.
 

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Discussion Starter · #13 ·
Took it to a shop we've had other exhaust work done at. Quoted $350 all in: bad flex pipe.

But what about the CEL code, you say? Beats me. As the wife drove it away from the house she said the code was gone.
 

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Took it to a shop we've had other exhaust work done at. Quoted $350 all in: bad flex pipe.

But what about the CEL code, you say? Beats me. As the wife drove it away from the house she said the code was gone.
Ya, those emission codes come and go. If you have the top end running smooth, the fuel trims are near zero and there is no oil consumption, it's likely the exhaust leak poping that code.
 
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