P0171 what to try..

Any amount of leak around the sensor can trip a code, I'd be surprised if that doesn't solve the issue for you

 

It didn’t. Drove about 25-30 miles and it came back.

 

Im guessing either a bad after cat o2 or the replacement cat has already went to crap.... it has 9k miles on it was it. My luck, i watched the live data and the second o2 is bouncing voltage instaed of being steady. The replacement sensor is working normally.

 

I'm not sure on this truck but on mine the upstream sensor is an air fuel ratio sensor and the downstream is heated 02. Do you have the correct sensor in the rear? It's hard to believe but not impossible that the cat is bad.The downstream 02 should be steady with the cat operating correctly. Can you check the inlet and outlet temp on the cat with a temp gun? There should be a pretty big difference, about 75 degrees, output higher. If the ecu sensed lean burn it increased injector pulse width to richen the mixture until it went out of acceptable range thus the check engine light. Did you drive it long with the light on and was it flashing?

 

I seems that she wants you to spend more $ on her... If it was me, and this has been me, I would buy a O2 sensor simulator from URD for the $100+ and install it, clear the codes and be done with it. If this is a smog state then remove the URD simulator just down the street from the inspection facility and drive it there. If the state requires a tail pipe test, you might have a problem.

 

In my odd P0171 case, the code presented shortly after I installed an aFe Pro Dry air filter in the stock airbox. I cleaned the MAF, no change. Then I cleaned the PCV valve. That was when the code went away for good (going on four years.)

Weird, but heck, cleaning the PCV is free...

 

A bad or leaking pcv valve or hose can give you a sizable vacuum leak.

 

Ive gotten rid of my 171, just not the p0420.

 

When the downstream 02 signal is looking like the upstream it is a signal that more O2 is exiting the cat than there should be hence "P0420 catalyst system efficiency below threshold.". The converter should be exhausting low oxygen as the combustion gases are converted into H2O and CO2. Do you have any other exhaust leaks that could introduce air into the system? It might be that the ecm does not like the signal from an aftermarket sensor but that's a long shot. If the tailpipe emissions exceed 1.5 times the federal standards it turns on the light. Does the exhaust smell rich?

 

No, not really rich. Matter of fact it runs great!
Ive checked and there are no exhaust leaks. A little backstory: A year or so ago the truck began to run funny, turns out it just needed some newer high quality plugs and wires. Problem solved, however the truck was already being serviced for a new water pump and timing belt, so the mechanic says oh your factory cat is clogged. So he threw a cheap replacement on it. Got the truck back with a small exhaust leak right after the cat that turned out to be from a stripped bolt. However, never a check engine light. Fixed the leak and it was good for over a year.. Got a cel of p0171 randomly one day. Looked at the o2 sensors and found them to be factory toyota ones, thought well the sensors are 20 years old might as well order a new upstream Denso for good measure.. Pop the hood next day and bam a busted vacuum hose, purchased some new put it on as well as installed the New denso (upstream) o2 sensor.. Next day clear the p0171 drive it for roughly 30 miles and cel comes back. I assume its the p0171, but this time its a p0420? Check the newly installed sensor and would you know its loose, not by much but nuts had come loose a bit.. Removed it, checked gasket and reinstalled extra tight. cleared code, comes right back within 25 miles. Looked at the live data and the (new) sensor is swtiching voltage like it should but the rear (old) sensor is going up and down as well. My thoughts now are either the ecu does not like the aftermarket cat, or the old rear sensor is toast. Just odd that even with a pretty bad exhaust leak it never tossed the 420, but i install a new denso upstream sensor and within 30 miles it throws the 420 with zero exhaust leaks. I just hate to put in another $50 sensor just to find out the new cat is the issue.

 

It seems like the the sensor is doing it's job though and telling you something is wrong with the cat. If you hook up the scan tool with the engine in closed loop and look at short term fuel trim you should see it go positive if you pull off that vacuum hose you found bad. That should tell you the upstream sensor is doing its job and the engine is responding. The downstream sensor mainly for monitoring the cat efficiency. A way to tell would be to look for a temperature difference with an IR temp gun at the inlet and outlet of the cat. That will confirm that a reaction is going on and it should be about 75 degrees. The temp guns are cheap at Harbor Freight and even Home Depot or maybe you can borrow one. There might be a warranty on the newer cat? Is the connector on the downstream O2 sensor clean and seated well and the wiring is not damaged?

 

Im leaning towards the cat myself. Just found it odd it decided to fail literally within 30 miles of replacing that upper o2. Its been on the truck well over a year with no issue then as soon as i replace the upstream o2 it gives me the p0420. I dont own a IR gun but it may be worth getting one. If it is the cat im not gonna replace it, no smog test where Iam. Just hate the annoying light. And the downstream looks to be clean and tight, wires are in good shape as well. Truck runs great, no issues. The short term fuel trims were good, plus or minus 2-4% at 2000rpm

 

The O2 level in the exhaust is measured by the downstream sensor as a percentage of the upstream sensors switching rate.When the downstream sensor switching rate gets close to the upstream it flags it and turns on the check engine light. The upstream should switch rapidly, the downstream should be fairly steady so a big difference. Your signals are similar so the check engine light gets turned on. The sensors are doing their thing, the cat may indeed be dead or dying. I wonder what happens if you put the old sensor back in.....

 

I kinda wondered the same thing lol. They are indeed both denso but they are 20 yr apart. The old factory toyota one has a bigger end with less air holes, i have a pic of them somewhere.

 

what happened to me I had below threshold code come up (cat converter ) in bank 2 I used 2 spark foulers drilled 1 out screwed it all in, reset the light and boom ! perfecto no codes ... works perfect ...***just a little used car salesman trick I learned****

 

you can use 2 spark plug foulers... drill the inside of the 1 out with 1/2 inch bit... screw the 2 together with the o2 sensor screwed into the one drilled out then the other fouled then into ur exhaust pipe reset code and your golden

 

What does the foulers do to the sensor to trick it?

 

it remeoves it from the exhaust stream...

 

it sets it back away from direct air flow but it still enough flow so it wont set off any check engine lights

 

That makes sense, on a good note the engine light just randomly went out today. Not sure if it will stay off but fingers crossed it does.