Welp first post trying to figure out an issue with my 2006 v8 4runner. I purchased with 201k miles last year and I’ve done a lot of engine maintenance last September and the cars been running fantastic until a few weeks ago.

I noticed a ticking in my exhaust on start up but can’t hear during idle. I was going to have a friend of mine inspect and weld were needed to resolve that issue. Also I have the check engine, vsc, traction etc on and I figured it was from the leak.

About a week later I noticed my mpg dropped dramatically. I use to get between 300-350 a tank and right now I’m getting around 150 or 7mpg. I get the codes scanned at a local parts store and I received P0174 system too lean a/f lean bank 2, P0420 Catalvst System Efticiency Below Threshold (bank 1)
P0430 : Catalyst System Efficiency Below Threshold (Bank 2).

I’m going to my friends shop tomorrow after work to inspect but wanted to get y’all’s advice on what to attack first. I’ve searched here and I’ve seen answers all over the board. But I can’t afford to get gas every 3 days at the current prices so I need this addressed asap.

Thank you,

You most likely need to replace both exhaust manifolds. They have the cat built into them and with over 200k miles, the manifolds have pin hole leaks causing the issues you are seeing and trying to weld them is not going to fix the issue. The manifolds are a double wall design and you will never be able to weld them up properly. The cats could be going bad as well due to age and high mileage. They don't last forever.

P0420 (driver side) and P0430 (pass side) are for the O2 sensors after the manifold cats. I'd replace both manifolds with OEM, not aftermarket. Replace the O2 sensors and both A/F sensors that are before the cats with Denso sensors or OEM.

This is something I need to do on my truck as well as I still have the OEM manifolds and over 337k miles. It's got the tick at startup, throws codes like crazy in cooler weather but hotter weather like we have here in Tucson during the summer, it's not an issue. I've already replaced the O2 sensors and all exhaust related gaskets. No other issue with the engine, it's had excellent maintenance. I have a buddy that's a Toyota Master Technician and he recommended replacing the manifolds due to the high mileage my truck has. He said the cats are probably failing just from age and high miles.

P0420 & 430 could be from exhaust leaks and/or cats not doing their job.

P0174 is the one I bet is affecting your fuel mileage and may not be related to the other 2. I guess it may be possible that a big exhaust leak above the upper O2 could affect things but it looks like the computer is seeing things too lean on one side so throwing fuel at the issue. Check out the thread below which has links to other threads. You may have to check fuel trims and such to narrow that last code down.

need some help: fault code P0174

Thank you guys!

looks like I'll need to throw a lot of money at this to resolve my complete issue. If I replace my o2 sensors now will that help get my fuel mileage get back to where it was? I can afford to do that now then hopefully manifolds and cats next month. I would like to do it all at once but I cant afford to throw a whole lot of money at it currently.

I agree that the P0174 code is more than likely what is affecting your mileage the most, look online for causes of that code and start there first, items like MAF sensor could need cleaning or replaced, a bad vacuum line leak, be sure to check the vacuum hose for PCV valve, dirty fuel injector, etc., etc., can a o2 sensor cause that code?, most likely not, here's a search links with some answers to get you started.

causes of code p0174 - Google Search

From the reading I did on this code, some believe it shouldn't be MAF or similar because it's limited to one bank. The OPs money may be better spent having someone with the right tools diagnose that particular code. One thing that could be tried, disconnect the upper o2 sensor on the offending bank and see if the mileage gets better. You'll get another code but the computer will not go into closed loop and should run on a preset air/fuel mixture.

I had a Jeep that would buck at a particular temp and I had no codes until I hit the remote starter one day and got an upper o2 code. The car ran perfect in open loop. Reset the code and restarted it the next day and it bucked as soon as it hit closed loop monitored with a Scangauge, code didn't come back but it gave me a starting point. I unplugged that o2 and it ran fine for a few days. Replacing that o2 fixed the issue.

I'm curious if the exhaust smells like raw fuel on this one.

My exhaust smells heavily of gas. If I sit at idle I can smell it strongly from the inside of my vehicle. Every now and then from a cold start my car will emit a white smoke immediately then go away.

Then I wouldn't delay figuring this out. Raw fuel is not good for your cats.

Thank you! Trying to get it resolved this weekend. Could this do any harm to my engine?

Hard to say but running enough extra fuel through it so as to smell can't be good for it.

Correct. If left for too long, the added fuel will get past the rings and cause the engine oil to start to thin out. So yes, over the long term, it is bad for the engine.