P0171/P0420 but Passed Emissions

Howdy,

New to the forum - Sorry if this is in the wrong section.

Purchased a gen 1 last week so I figured I'd make an account.

Brief overview of the CEL issue I'm having - Drove the vehicle 1200 miles back from California last week. Vehicle spent it's entire life at sea level and on the way back it threw a P0171 code (Lean - Bank 1). This was after gaining approximately 6,000 feet of elevation over 400-500 miles.

Upon inspection, MAF looked good - no obvious gunk or debris. The adhesive that was used to bond the two "filaments" together was dark and possibly discolored. After cleaning the MAF and clearing the code, the CEL stayed off for the remaining 700 miles of the trip.

I averaged about 15-16 MPG driving 75-85 MPH the entire trip. There are plenty of steep climbs along I-80 East and given the speed I was traveling, I figured the MPG wasn't terrible but not great for this truck (19 MPG highway?)

After arriving back in Colorado and driving for a few days, I took the vehicle to get the emissions check and it passed with zero issues.

The next day, during my drive back from work, a P0420 code appeared (Catalytic System Efficiency Below Threshold - Bank 1). The scan tool I was using highlighted O2 sensor B1 S2 having voltage range for 0.00 V to 1.24 V (seems too high/too low). STFT and LTFT at time of CEL are -2.3% and -6.3% respectively.

I know that people mention a clogged cat, possible exhaust leak or O2 sensor being the culprit for the P0420 code. Considering I passed emissions, a bad cat seems unlikely? I plan on testing the downstream O2 this weekend and testing for exhaust leaks. This O2B1S2 seems like it could be the issue.

Based off service records, this code has occurred before. Exhaust system (MAF and O2 sensors included) was replaced by PO in 2017.

Truck idles great at ~650 RPM, drives fine and fires up with no issue.

Curious what others have to say.
Appreciate any help I receive.

 

Could be a number of things.. typical culprits are vacuum leak somewhere, fuel pressure, maf/o2 sensor. Could even be ECU "re-learning" due to thinner air.

 

p0171 is a hard one to pin down because there can be so many causes for it....start looking for vacuum leaks as that's a fairly common one, especially with these older trucks/engines and brittle vacuum lines. Clean the MAF sensor

 

I noticed one of the vac lines to my evap system was starting to split approx 3/8" from the end. The hose is good back where the mouth of the nipple is. I'll do another check of the vac system.

 

P0420 in my experience was only resolved after replacing the catalytic converters, which unfortunately right now are extremely expensive if going OEM. There are a number of cheaper aftermarket options for 1st gens though, I got Magnaflows several years ago for $600 installed (50 state legal). Probably a lot more now though.

An exhaust leak before the O2 sensors can trip that code as well, so check for those

 

I should add, when I got my P0420 it came and went for awhile before staying permanently...when you passed emissions I'm guessing the system cleared and the cat monitors were good/green which is why you passed, but on a later cycle they came up as being bad again and the code came back. It will be like that until the cats get bad enough that the code just stays on

 

Based off the PO services records, the exhaust and cat was replaced back in 2017 (20-30K miles ago if my memory serves me right). Seems crazy that a CAT could go bad that quickly. Unfortunately, there's no indication on the document what brand was installed.

I'll go through all vacuum lines; check exhaust for leaks; test O2 sensors. Thanks for the help, Blue. If anyone has experience with this feel free to chime in. I'll update the thread as new information becomes available.

 

yeah unfortunately if they purchased some really cheap aftermarket cat/cats, then they could easily go bad fast. Or not "bad" per se, but not be good enough to satisfy the O2 sensors which causes them to trip a code for inefficient catalyst.

 

An often overlooked maintenance item that can cause what is effectively a vacuum leak is the PCV valve. Make sure yours is functioning properly. Also, fuel trims long and short during idle may help you figure out the issue.

 

Thanks for the input. I will definitely check this.

Do you know what I should be looking for specifically when it comes to long and short trim values?

And for redundancy, OEM PCV GROMMET 90480-18001 OEM PCV HOSE 12261-62041 OEM PCV VALVE

$16 Genuine Toyota 12204-62010 PCV Valve Sub-Assembly

12204-62010

 

ideally you want your fuel trims to be in the -10 to 10 range, closer to zero the better. Too high, too lean...too low, too rich

I suspect yours are up above 10 given the lean code but I'm not sure how high they have to be to trip the code

 

Quick update - Got a few metrics from my scan tool on the commute back from work. Very mellow drive, lighted city streets. Did not exceeding 35mph and keeping the vehicle RPMs below 2500. It's snowing heavily right now so I won't be able to check sensors/vac leaks until this weekend.

RPM Range - 585 to 2450 RPM
O2 B1S2 Short Term Fuel Trim - N/a (Not sure if a metric is usually available for this reading)
O2 B1S2 Voltage - 0.060V to 0.860V
MAF Rate @ idle - 0.6 lb/min (seems extremely low; I plan on picking up a MAF after seeing this. 3.4lb/min is standard at idle)
MAF Rate Max - 3.4 lb/min
Long Term Fuel Trim B1 - (-16.4% to 19.5%) *Definitely an issue. Never settled around 0% unless throttle was held constant. When accelerating deep (+) values, when coasting deep (-) values*
Short Term Fuel Trim B1 - (-20.3% to 7.0%) *I never saw this minimum value occur. During the drive it floated around 0% most of the time. I'm thinking the low may have been on start up?

 

Update: Checked through the engine bay for vacuum leaks using propane. Nothing jumped out. There were two hoses connecting to the EVAP system that were beginning to split, as well as the PCV hose. I plan to replaces these tomorrow.

Next, I checked the resistance of the upstream and downstream O2 sensors. Both checked out to be within spec. I believe they were 13.4 ohms and 0.8 ohms, if my memory serves me correctly.

Following O2's, I checked for exhaust leaks. Found a major leak at the downstream sensor flange, as well as a small pinhole at the first cat, just before the flange. Picked up a new gasket for the O2 and discovered one of the studs didn't even have a nut on it and the other was either rusted to oblivion or welded to the sensor. See photo.

I ordered a new flange kit from YotaShop to cut the studs and weld in a new flange. I'll fix the pin hole for the first cat as well.

I also checked the temperature at the 2nd cat intake and compared it to the exit. I noticed the intake was are 290*F, while the exit was only 230*F. From what I've read, if a cat is working properly, the exit should be 30 to 100 degrees higher in temperature than the intake. This will confirm the chemical reaction is happening properly. But then again, maybe due to the downstream O2 sensor leak, there isn't as much residual heat as there would be. Or maybe my cat is clogged?

Does anyone have experience attempting to remove studs with vice grips? I'm thinking I might give it a shot after work - let me know if you have any tips other than penetrating oil and maybe grinding in a couple of flat sides.

 

I had an issue with a broken stud on the upstream sensor. @Blackdawg suggested a stud extractor. Ordered this one

$27 ARES 70016 - Damaged Bolt and Stud Extractor Tool - Grips and Removes 1/4" to 1/2" Studs - Ideal for Broken, Rounded Off, Painted Over, and Rusted Tight Bolts

https://smile.amazon.com/dp/B01GKEBG9I which is currently out of stock but others are out there. Had no issues at all getting that broken stud out.

Should do the same for your situation. Order up an extractor and hopefully you'll be laughing at how easy it was.

 

Parts store should just have these too. Where I got mine.

 

Right on. There's a tool for everything. I was planning on trying the double nut method but I'm always down to spend $20 on the proper tool. Thanks, Mike!

 

Picked up a stud remover. Due to the shield that was protecting the downstream O2 sensor, it wouldn't fit unless the shield was removed. I was able to bend the shield up and over the stud that was missing a fastener and discovered that the weld was actually a nut that was rusted to hell and back. Shifting the heat shield back a forth, I was able to break the fastener loose and thread off what was left with a pair of vice grips.

The O2 sensor looked like it was in great shape w/ in-spec ohm reading, so I chased the studs with a die and the new gasket was installed - No more leak. Truck seems to have a bit better pick up and the CEL hasn't illuminated yet.

Does anyone know what value the MAF rate should be at idle for a 3.4L? I haven't plugged in my scan tool since the O2 gasket was replaced, but I was getting 0.6 lb/min at idle. Seems very low based off the little information I was able to find. Air filter is brand new.

Edit: Also replaced PCV hose. It was on it's way out. Planning on doing the valve and grommet once they're delivered.

Here's a photo of the nut for entertainment.

 

Solved the fuel trim issue - The P0171 code popped up again. Cleaned my MAF for the second time with no change to fuel trims. Swapped in a new sensor and it seems to have solved the problem. Now my fuel trims stay in the -10 to 10 percentile. Truck feels more responsive accelerating from a stop. Haven't been on the highway yet. Hopefully MPG will improve now.

As for the P0420, it hasn't returned since installing a new gasket on the downstream O2 sensor. I will follow up to let everyone know if anything changes.