Actually, that is not my recommendation. I use the LARGEST orifice, not the smallest. I suspect any of the orifices would actually work fine, or for that matter no orifice at all. The effect of simply moving the sensor farther away from the exhaust stream does the trick.

As I mentioned, the rear sensor has a very limited function. Monitor the cat monitor if it is going bad by detecting the amount of o2 after the cat and comparing it to the amount before the cat... It wants to see a drop, but still detect some o2. Simply moving the sensor out of the main exhaust stream is usually enough to avoid a bad cat CEL. However if you move the sensor too far away, or use too small of an orifice, or simply don't mount the sensor into the exhaust system at all, and it sets another error code.

It reads to little o2, which the computer interprets as a bad sensor and can trigger a 'bad secondary 02 sensor' CEL.

No guarantees in life, but I suggest using the largest orifice, not the smallest. If you get no ECU error messages, you are good (the likely case). If you get a 'bad cat' error, you are sniffing too much o2 and need a smaller orifice (unlikely). If you get a bad rear o2 sensor error, you are not sniffing enough o2 and need a larger orifice, or in fact have a bad o2 sensor.

 

Vibrant was the brand that designed and made the first O2 extenders of this 90 degree style with changeable orifices that I had seen and used, so I refer to them as 'Vibrant Style'. I'm not sure if they are still producing them, they don't show up on their web page.

A bunch of look-alikes have come on the market, but they always seem to be under a new brand every time I look for them. Some on eBay are listed as Vibrant brand, they may or may not be actually made by Vibrant, but it doesn't really matter, if it is the same style they should work fine.

Example:
https://www.ebay.com/itm/153764143915

 

I'm not an expert, I'll give my .2 cents and my "car guy science".

I've run two different 991's with bypass pipes and extenders. Both got CEL's intermittently.

Both cars would trigger the CEL when the car would be slowly moving or idling in traffic for extended periods. My car guy science says that this is due to the exhaust gases dwelling in the pipes rather than being pushed out which make the gases creep up into the o2 extenders thus triggering the code.

The o2 extenders are 50/50 IMO.

 

Used to be the case, and I'm certainly not a P-car ECU expert, but many newer platforms also use the rear O2 for LTFT, as well as cat efficiency.

 

Interesting...

Can I ask what extenders you are using? If they have changeable orifices, which ones? And what error codes you have gotten?

I don't spend a ton of time in stop-and-go freeway traffic, but I don't currently get even occasional o2 or cat related CELs in either my 996 or 991 turbos in the past year with cat bypass and extenders.

I will note that on the 996, after first installing the cat bypass and extenders, I would get an occasional CEL pop with:

- P1117 bank 2 rear
- P0139 bank 2 rear
(Both drivers side rear O2 sensor errors.)

I swapped the rear sensor right to left, and began to get occasional:

- P1118 bank 2 rear (same as 0117 but for passenger side)
- P0159 bank 2 rear (same as 0139 but for passenger side)

The error moved with the sensor. I replaced the single sensor and haven't had a single CEL since.

Now the occasional CEL's did start right after I went catless with the extenders. Did I perhaps slightly damage a sensor during the installation? Was the sensor marginal even before the conversion and didn't have enough headroom to avoid CELs in a catless environment? Don't know, but a new $85 sensor made the problem go away.

 

I've used the ones that Fabspeed sell with their pipes, and the ones that Soul sell with their pipes... I just got my CEL on my TTS running the soul pipes and extenders but have not checked the code... But I will say the CEL triggered exactly the way it did on my old car... On my first 991 with Fabspeed extenders and pipes, there were times I wouldn't get a CEL for 2-3 months then bam! CEL... Sometimes the CEL stays for a week or two, and sometimes it goes away on it's own the same day.

Interesting info you laid out though. I will have to read the codes with my AP and report back.

 

Not an expert either, but pretty sure rear o2 sensor on 991.2 or earlier are not used for fuel trims... LTFT or otherwise. No idea on the 992 platform.

 

Yeah, Fabspeed sells the extremely pricey 'mini-cat' style extender. I haven't heard great things about them and can't speak for how reliable they are. Knowing what the rear o2 sensor does and what it is looking for, it does seem a bit silly to think an actual mini-cat' would be of any advantage.

See what errors you are getting, and if they are indicating 'bad sensor' or 'bad cat'. See if you are getting errors on both sides, or consistently on a single side. Lastly, I'd suggest swapping the fabspeed $300 'mini-cat' extenders with the simple $30 90-degree extenders shown above and buy me a beer if your problem goes away.

 

The Fabspeed ones I was running was on the old car. Now I'm running these




I would take your advice, but I should be getting a nice big box from Kline in another month to month and a half.

What's your opinion on the soul o2 extenders?

 

^^^ Those are the style I like. I don't know if Soul actually makes them or just resells them. Pretty simple parts.