MAS showing 1.14g/s with engine off?

Long story short. Had a p0420 and p0430, went to get cats changed. Ran a diag at the shop fella said to change the MAS it was bad it showed 1 g/s when the truck was off. I changed the MAS, all codes vanished for 3 weeks, then a p0456 popped up. Changed the o-ring seal at the vapor valve at the evap canister. I changed the part out yesterday completely and on the way into work the p0430 popped up again. Ran a live data OBDII and the MAS g/s is all over the place from 4% - 99% while idle, driving or highway.

MAS showing 1.14g/s with the engine off?

Bad ground?

you are talking about a mass air flow sensor (MAF) as it reports in g/s. check the voltage as well.

it does make a difference, what type of car are we talking about... yr, make, mdl, engine.

2010 Nissan Titan SE

I dont have live data for voltage but will see if I can get that on the drive home.

My understanding the MAS would be 0.0 as the engine is shut off

their are a lot of variables here... your mechanic changed cats only off the P code and did not tell you about possible problems with your 02's which caused the cats to go, I have to say hmmmmmm... your truck must be running extremely rich, that's what leads to cat failure. Running rich is either a mechanical problem or an 02 problem. Can't figure why your not getting other P codes with a problematic MAF and burning out cats. You might have a loose connection at the MAF.

Generally your g/s will be engine liters (4.0 or 5.0 ect) at idle. With the engine off, could be your scanning device.

Oh sorry. we did not replace the cats. I replaced the MAF after my mechanic scanned it. He was the one that suggested it was a bad MAF, he was also the one who saw that the MAF was still reading air volume with the engine off. He used a SnapOn scanner, I have a cheaper Launch reader. No other P-codes, but last week was P0456 and then the P0430 again this week.
I just looked at some recorded live data and the MAF was running at 77+g/s.

I have a feeling loose connection maybe a factor, I had an issue with my ABS unit and it was a ground problem. What runs off the same portion of the harness? the MAF

I will upload the live data recording.

Thank you. So me just sitting in the truck, key on, engine not running I am showing 1.14%.

what does yours read without the truck running?

Here’s diagrams.

I agree that it may be a wiring issue.

I would test by other means than a scan tool as you should never completely trust your scan tool data.

I’d be curious and put a fan in front of it with KOEO and see if the MAF value changes. Not high tech but sometimes simple things give good direction.

Oh sorry I thought you were reading data from the 2008

Interesting, I wonder if you pull any reading with the key on? if it's 0 then I would assume I need to be at 0 too.

will do

Did not check voltage at the MAF today it was a bit cold, windy and snowy. I did get some live data to show and interested in the feedback. Thanks!

The feedback I got today was cold air intake caused the truck to run super lean and trash the cats.

UGH, I will make sure I test again with the cats hot, I am pretty sure they were but I am in denial about replacing the cats. On another note, my check engine light turned itself off yesterday.

So cat replacement, here I am again. How do you feel about this shop that quoted my up to 1000 to cut out and weld in a new set of cats? Do these aftermarkets cats work? My guess will be a set of Magnaflow's.

the sensor reading you are seeing are air fuel sensors... they will be a steady voltage reading. for nissan its 2.2v. the important trim to look at are the LTFT's. your STFT are neg 21 and 17, but its important to know what the long term trim are.

don't get too worried just yet about the cat's... that MAF sensor can cause what you're seeing. unplugging the MAF is a test you can do, but it's not full proof. when plugging the MAF back in, turn off the engine and key off for about 5sec before restarting. this will reset your MAF pid so it reports properly again.

your engine is getting too much fuel so the pcm is taking it away in an attempt to return to stoich.