Neighbors OBD readouts Help
 

My neighbor asked if I could help read these from a local wrench. I cannot.

His check engine light came on and the shop connected their OBD reader and printed these reader charts for him. Says it tells them he needs new CATs and O2 sensors and for 5K they will do the work. Car is a 2005 Nissan Pathfinder.

What is this telling them? Anyone have anything to share? Thanks
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I'm not a mechanic (shade tree type only), but, I'd ask to see what a 'normal' readout looks like before laying down that kind of money.

Are there actual OBD codes being thrown?

How many miles on car?

Is it a P0420 code?

I had a bad CAT in my Taurus. I got very erratic readings from the downstream 02 sensor. IIRC, the 02 sensor signal should be somewhat flat. If it's bouncing all over, you start by replacing the 02 sensor because it's the cheap starting point. If the new 02 sensor doesn't fix the problem then you look at the CAT. I bought a new CAT for my TaurusX for $127. Installed it with the old O2 sensor and it has been fine ever since.

My kid had an Audi A4 that would give codes for start up values for the O2 sensors. Apparently, those only occur at start up and have zero to do with running after warm and won't throw a code any other time than startup. The recommended repair was new cats...

There are some companies that sell extenders for the O2 bungs to get the sensor further from the pipe where the readings can be wrong. I installed them and they worked like a charm.

Assuming this is similar you may want to look it up. From your post it seems like it is the issue.

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Looks to me like they are illustrating the mismatch in the output of the secondary O2 sensors compared to the AFR to point to catalyst inefficiency? But I’m no expert in advanced diagnostic stuff.

I would start with the secondary O2 sensors, they are about $100 each and shouldn’t be more than an hour of shop labor. A failing secondary O2 sensor will indicate a failing cat. If that doesn’t fix it I would have him go to a custom exhaust shop and quote for an aftermarket cat replacement. If they are quoting $5k it’s probably for new OEM cats. Or find somebody to remove the cats and weld in some pipe for cash on the side.:)

From what I am reading 125% is too high for the ALPHA numbers. I don't fully understand but the readings have to do with the upstream AF ratio sensors. Not the cats.

Downstream sensors are reading rich at the almost one volt reading - I think that's what that means if they are like traditional downstream sensors.

Faulty/dirty Mass Airflow Sensor might be confusing the control box directing it to dump more fuel than required?

Reading below hurts my head.

https://www.nissanforums.com/threads/how-to-read-consult-printout.41277/

Would be helpful to know if there is indeed a cat efficiency code and whether it is a (hard) code or soft? also, what is, if any other codes present?

Thanks for the replies. I'll see if he has the actual codes.
I too would like to know what normal readings look like.

Thanks again.

It's called a spark plug non fouler.
https://www.amazon.com/Dorman-42009-Spark-Plug-Non-Fouler/dp/B000CO91FY

^^^^no the better ones have a screen / diffuser in the bottom^^^^^^^

I don't remember if that is what I bought. I have seen some that claim to have screens at the end that they describe as mini CATs.

For our Audi, the codes were only read at start up and were irrelevant later as though those O2 sensors were useless other than to throw start up codes. Fairly certain it was the O2 sensors upstream in front of the CATs.

By tricking those O2 sensors the CEL was cleared and the PITA Virginia vehicle inspectors were happy.

Could be. Don't know what the point of it would be. The non fouler just gets the o2 sensor out of the direct flow of the exhaust gas. Or at least that was the purpose most claimed on the TaurusX forums. I never did it. When I found out I could buy a new direct fit CAT for less than $150, I figured why not just fix the stupid thing.

Technically, those 02 extenders would fail Pa state inspection. The last ones I saw, had a small hole in them to limit how much of the exhaust gasses the rear 02 sees .

Years ago, I replaced the exhaust on my Prelude with one without a cat or sensors.
To avoid codes being displayed, I bought an 02 simulator from ebay. I think it was $19
It's simply a resistor that is spliced into a couple wires at the ecu under the passenger floorboard.
It's worked for over 20 years now with no codes being shown.

CA as well. Some smog techs look harder than others.

Luckily in Virginia they didn't look but in my case they were behind engine near the firewall. Very hard to see and pretty sure you couldn't see from below. But that also made them a PITA to install.

I have since moved to the land of freedom where they only check lights, horn, and wiper blades.

There are Youtube vids on how to wire a resistor and a diode into the rear O2 sensor wires and wrap them back up with the corrugated plastic shielding to hide the illegal override. It works but if a real shop did it there could be hell to pay if they were busted.

As noted, CEL codes would be nice to have from OP.

Buy a code reader and plug it into the OBD port.

Your FLAPS might even rent one out for you for free ...

I'm still in the camp that a dirty MAF is a highly suspect part. Intake air passes over the wire and there is dirt on it and it does not cool enough. ECU thinks it needs less gas than it does. Front O2 sensors pick up the exhaust is lean for operating conditions and ECU adjusts by pissing in more fuel.

There is a problem upstream due to the high A/F Alpha percentage (125%) and it has nothing to do with the downstream O2 sensors. Spark plug foulers, diodes, resistors, etc will not fix this.

There may be an intake vacuum leak that impacts both banks as well.

Here's a decent write-up of the A/F Alpha which is Nissan's term for long term fuel trim, I believe.

^It's not that the car is necessarily running rich. The alpha correction indicates that the car has decided that the stock fuel map is not producing the proper mixture via the feedback loop, and the car is compensating by adding 10% (in the case of alpha = 110% for example) to the stock fuel map at the particular running condition.

To put it another way, say you have a completely stock car that is running perfectly. The alpha value when cruising at a particular RPM should theoretically be 100%. This means that the stock fuel map is producing a perfect fuel mixture which needs no correction.

Now lets say that you've just added an intake and an exhaust. There will be more air than the car is used to seeing at a lower throttle position, and thus the ECU will find itself at a different part of the stock fuel map. However, it may find that the wrong mixture is coming through, and produce a correction for it. This is where the alpha value comes in. It will enrichen the mixture if it's above 100%, or lean it out if it's below 100%. If the computer finds that it's making the same correction over and over again, it will eventually change the stock fuel map and the alpha will return to 100% with the new fuel map in place (this is called the long term fuel trim).

For this system to work properly, you need all of your sensors to be in tip top shape. If you have a bad O2 sensor, or a bad MAF (for example), your alpha value will be wrong and the car will make improper corrections to the fuel map, which could result in running too rich or too lean.