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if your handy with a welder you can piece in a section of same diameter pipe or i.d. over o.d. and clamp it recommend band clamps they are like 4 inches wide and have a better seal capability
You changed the exhaust manifold gasket. Did you take a look at the exhaust pipe, the end connections into the exhaust manifold, and the connections into the catalytic converter? There could be a small hole that that is large enough for the sound to be heard. If you don't know what to look for, have a mechanic look over the entire exhaust system for you. You may be surprised at what they may find.
look near throttle body for backpressure transducer and mechanical egr valve, bpt burns up if plugged causes exhaust leak.e g r will need replacing also.
Look for a crack in the block or head, a blown gasket, or a bad expansion plug*. I assume the water is your coolant. If that is the case, the level in the radiator will be low. The coolant leak could be up higher - perhaps it's coming from the thermostat plumbing or the temperature sensor. It could even be a crack in one of the heater hoses. The worst case is the cracked block - in that case the engine is essentially junk. A leaky hose connection is the easiest to fix. *Note on expansion plugs (AKA freeze plugs). These look like inverted jar lids pressed into the block and are designed to be pushed out if the coolant freezes so the expansion of the ice doesn't crack the block (freezing can happen if you don't have a proper anti-freeze/water mix in cold weather). If you don't replace your anti-freeze according to the maintenance schedule, they may rust out and cause a leak.
p0171 is lean exhaust bank one ( passenger side) and p0174 is lean exhaust bank 2 ( drivers side). First we need to look for vacuum leaks, low fuel pressure, Intake air leaks after the mass air flow sensor, pvc system. Because you have both sides lean I would start looking at these spots.
Before you invest in a new catalytic converter get the exhaust leak fixed, especially if it is before the cat and O2 sensors. An exhaust leak will allow air in and the sensors will not read accurately. This could show up as a code. Get the codes cleared after the leak is fixed and see if it comes back. It probably won't. Hope this helps.
Here's what the code means, P0420 -- Catalyst System Efficiency Below Threshold (Bank 1).
Causes: Bad O2 senor, bad catalytic converter, and/or leaking exhaust manifold.
The error is coming from the O2 sensor number 1 between the exhaust manifold and the catalytic converter. The sensor itself may not be the problem. From what your describing is maybe a leak coming somewhere near the exhaust manifold. This would set off the P0420 code. Look around the exhaust manifold down to the catalytic converter for leaks that will leave black marks from burned fuel. If this can't be easily found, you may have to take the car in and have the exhaust hooked up to a smoke machine. This will inject smoke into the tail pipe and any leaks will be found by the smoke coming out of it.
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