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MAS showing 1.14g/s with engine off?

  • 9w1911
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19 Mar 2020 12:53 #38446 by 9w1911
MAS showing 1.14g/s with engine off? was created by 9w1911
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?

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19 Mar 2020 16:20 #38451 by scannerjohn
Replied by scannerjohn on topic MAS showing 1.14g/s with engine off?
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.


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19 Mar 2020 16:25 #38452 by 9w1911
Replied by 9w1911 on topic MAS showing 1.14g/s with engine off?
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

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19 Mar 2020 18:17 - 19 Mar 2020 18:19 #38453 by scannerjohn
Replied by scannerjohn on topic MAS showing 1.14g/s with engine 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.


Last edit: 19 Mar 2020 18:19 by scannerjohn.

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19 Mar 2020 19:11 #38458 by Monde
Replied by Monde on topic MAS showing 1.14g/s with engine off?
I am looking at a 2008 Titan, the output voltage at idle is 1.0-1.3 v and 3.0-9.0 g/s at idle and 9.0-28.0 g/s at 2500 rpm. I am not at work,so I can't get you a wiring diagram for the 2010. It may not be different though. I would check for loose connection as suggested by Scannerjohn or wiring integrity while shaking the wires.

Never assume, diagnose!

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19 Mar 2020 20:23 #38459 by 9w1911
Replied by 9w1911 on topic MAS showing 1.14g/s with engine off?
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.

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19 Mar 2020 20:24 #38460 by 9w1911
Replied by 9w1911 on topic MAS showing 1.14g/s with engine off?
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?

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19 Mar 2020 20:51 #38461 by John Curtis
Replied by John Curtis on topic MAS showing 1.14g/s with engine off?
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.

Thinking out loud always helps me in the process.
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19 Mar 2020 21:26 #38463 by Monde
Replied by Monde on topic MAS showing 1.14g/s with engine off?

9w1911 wrote: 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?


I am not given a spec for the MAF with key on.

Never assume, diagnose!

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19 Mar 2020 21:28 #38464 by 9w1911
Replied by 9w1911 on topic MAS showing 1.14g/s with engine off?
Oh sorry I thought you were reading data from the 2008

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19 Mar 2020 21:38 #38466 by Monde
Replied by Monde on topic MAS showing 1.14g/s with engine off?
I am, but I am not given a spec with key on. I am using alldata. My program is up to 2009.

Never assume, diagnose!

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19 Mar 2020 21:51 #38469 by 9w1911
Replied by 9w1911 on topic MAS showing 1.14g/s with engine off?
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.

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19 Mar 2020 22:06 #38470 by Monde
Replied by Monde on topic MAS showing 1.14g/s with engine off?
Check the voltage at the signal wire. It should be 0,4v with key on.

Never assume, diagnose!

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19 Mar 2020 22:08 #38471 by 9w1911
Replied by 9w1911 on topic MAS showing 1.14g/s with engine off?
will do

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21 Mar 2020 02:19 #38505 by 9w1911
Replied by 9w1911 on topic MAS showing 1.14g/s with engine off?
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.

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21 Mar 2020 10:15 - 21 Mar 2020 15:57 #38513 by Monde
Replied by Monde on topic MAS showing 1.14g/s with engine off?

9w1911 wrote: 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.


I can see that truck is running rich and the rear o2 is switching back and forth. The rich condition may be due to the MAF sensor overreporting the airflow. Have you tried unplugging the sensor and seeing if the fuel trim is back to normal? The switching of the rear o2 suggests that the cats are bad because they are not storing sufficient oxygen unless the cats were not up to temperature.

Unplugging the MAF sensor may be not a good test since the pcm will use substituted values.( Back up strategy).

Never assume, diagnose!
Last edit: 21 Mar 2020 15:57 by Monde. Reason: Add correction.

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21 Mar 2020 11:21 #38521 by 9w1911
Replied by 9w1911 on topic MAS showing 1.14g/s with engine off?
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.

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