p0420 code, heavy exhaust smell, bad mpg, rough idle

Doing some research on the P0420 code. Seems like it could be one of the o2 sensors on the exhaust, could be the exhaust itself, could be a bad fuel pump? Where should I start first?

 

O2's

 

Take it in and have a diagnostic done.

 

Nah, don't do this OP.

 

$61 Denso 234-4189 Oxygen Sensor

Denso 234-4189 - downstream oxygen sensor

$143 Denso 234-9001 Upstream Air Fuel Ratio Oxygen Sensor with 10.6” 4-Wire Harness, for Auto Trans Equipped Vehicles

Denso 234-9001 - upstream air/fuel sensor

 

Yea, replacing one sensor will fix all that.

https://www.tacomaworld.com/threads/addressing-some-common-misnomers-of-the-p0420-code.396763/

 

Replace whatever is cheapest first.

 

Throw away enough cheap money and is not cheap anymore.


All the downstream sensor does is check how the cat is functioning. It doesnt control anything ahead of it, just tells you the is an issue somewhere upstream. If i have a hole in my leg j don't go buy new socks because they're cheap and mine feel wet.

 

FSM says: 1-Check for exhaust leak, 2-Check A/F sensor (bank 1, sensor1), 3-Check oxygen sensor (bank1, sensor 2), 4-Replace 3-way cat. There is a diagnostic procedure if you have a scan tool with freeze frame data.

I have had this code, but usually with other codes. Have had exhaust leak, and bad sensors. Haven't has to replace the cat yet (160K).

 

Diagnose the problem and then correct the problem..

Check your fuel pressure and then go from there.

 

Well obviously, yes.

Many times however, there are several possibilities for a given symptom. If you've got a $10 part that's easy to replace that could be the problem, might as well throw it on there and see if it fixes it.

I'd rather that then going straight to the $600 solution first only to find out that's not the problem.

 

Or do the proper diagnostic, checking fuel pressure, exhaust leaks, o2 resistance/heater voltage, etc, and replace the correct part.

 

I don't disagree.

But most post people don't have all the equipment/tools/mechanical know how to check all that stuff "properly", which is probably why they're on the internet asking for advice.

You work with what you have. You have to weigh the cost/benefit of doing those detailed tests, too. How long does it take to test it vs. how much does the diagnostic tool cost vs. just replacing a part that could probably stand being replaced anyway.

If there's a good chance the problem is a $10 part, what's the bfd in replacing it? Hell, many shops I've seen will do that, too.

 

I would rather give correct advise and offer my tools and workspace to help someone correct a problem than give them advise on a half-ass fix. The BFD is that a lot of times replacing an o2 sensor is just masking the true problem. Lots of shops will do that because they are shady and want you to come back, and know you will.

 

Check for exhaust leaks, check on youtube for various methods. Get yourself a Fuel pressure gauge with the correct fittings to check the fuel pressure on your vehicles. If that's not feasible for you check with your local TW guys, someone probably has the tools and is willing to help you out.

 

Another thing to keep in mind is that the O2 sensor and A/F sensor are actual wear items. I don't recall off the top of my head, but I think Mr. T says to replace them every 100k. If these haven't been replaced, most likely with your mileage (not sure what you're at on the odometer) they should be replaced. So it's not a bad idea to replace them. If you haven't already done them, you wouldn't just be throwing money at your truck and hoping for a fix.

 

Mine wound up being an exhaust leak, where the headers meet the 1st cat.
I found that out AFTER I bought new cats and O2 sensors and god knows what else to fix this problem, lol.
Check for any leaks first...please.

 

I've seen egr valves slightly stuck open cause similiar issues.

 

Options: throw parts at it and hope something sticks, spend a ton of money.

Or...

Pay 89.99 for a diagnostic and get it right the first time. And spend less money

 

You have faith in the 89.99 diag fee?