Oxygen sensors and catless down pipes

B6/7 S4/RS4
#1

So I posted this question in the DIY thread, but am going to ask the question again here to keep the DIY on topic. Mods you can delete those posts if you want.

Copy/Paste:

If I am installing catless down pipes and JHM tune, are the o2 sensors really doing anything? I know the rears just measure cat efficiency, so those shouldn’t really matter with the tune, correct? Are the fronts still working as they should? I have a CEL for B1S1, and am wondering if I should replace both front o2 sensors before flashing the tune or if it will override the sensors? I just don’t want to drop $4-500 for all four o2 sensors if I don’t have to.

justincredible responded with this:

[quote]I would suggest starting a post on this in the main section. But anytime you have a S1 code you need to replace the sensor. Those are the more important ones. The rear 02s control more then just checking to see if you have cats. They help test the health of the primary 02s they also help with fuel mileage and a few other smaller efficiency things. They are important. Of you change a sensor on one bank it’s a good idea to change the other one to.
[/quote]
So my follow up question is this:

Do you mean the same sensor on the opposite bank? or sensor 2 on the same bank?

I know the front o2s are the most important, I want to make sure everything is up to snuff before I purchase/flash the JHM tune and I want to do the o2 sensor(s) when I install the trexturk dps I have in the storage shed. I just don’t want to be spending any unnecessary money since I spent way too much Christmas shopping. The dps I bought used and they came with a 90 degree spacer on the rear o2 port. My mechanical background comes from the off-road side of the house and emissions stuff usually gets ripped out so I am new to this. I apologize in advance for any seemly obvious questions.

#2

It sounds like he meant same sensor opposite bank. You won’t need the 90 degree spacer on the S2’s as JHM has their tune written so it does not throw a CEL.

I would replace both fronts. If you ever need to replace the rear s2’s you can get the univseral from bosch(JHM sells them for $50), it is by far the easiest wiring I have ever done on a car. It comes with everything you need to connect the wires. Tool wise, all you need is wire cutters, and wire strippers. Took my 20 minutes to do from the time I pulled the car onto a lift until I drove off the lift

#3

JHM’s tune does not “delete” the rear O2 sensors like some tunes will for catless exhaust, but rather the parameters for acceptable range is modified for the catless flow.

So you do not need the 90 degree elbows with the rear O2’s, but you do still need the sensors.

Based on your error code you definitely need to buy one new front O2, and while it is a bit pricey, it might be nice peace of mind to replace the other front O2 (but not mandatory).

Also, be aware, offer with older rear O2’s, once you go catless, the extra flow will finally kill them off. So you may have one of the rear O2’s fail after the DP install, but as mentioned above, you can just wire in a new one for cheap if that does happen.

#4

I had a feeling he meant both fronts, but I figured I would get clarification.

Also, I must have missed the universal rear ones when looking on JHMs website, $50 is a lot easier to swallow than $125 for the OEM ones.

#5

I bought mine through Summit Racing Equipment. I live 30 minutes from their store and I have a Proshop account so it ended up being cheaper than shipping it from JHM.

http://www.summitracing.com/oh/parts/bch-15733/overview/

#6

JHM’s listing:
https://jhmotorsports.com/shop/catalog/o2-sensor-rear-universal-for-b6-b7-s4-p-697.html?cPath=21_56_61_240

#7

Yes if you replace the front driver side 02 replace the front passenger side at the same time. They tend to ware at the same rate. It usually isn’t too long before the other side fails. And it’s good to have a good sensor in both banks for keeping the motor running well.

#8

Quadfreak, the front O2s are the most important, but the rears do a little more than cat efficiency so you can’t do anything crazy like delete them completely, even with a tune. I believe they are still used for long term fuel trim…Justin can probably say exactly.

#9

Yeah I got both front o2s on my list of stuff to order. Along with PCV, spark plugs, air filter and intake spacers. Anything I’m missing as far as preventative maintenance?

#10

Valve cover gaskets if they are leaking? But I don’t know if you have done that previously

#11

Yeah the PO did the VC gaskets before I bought it. I also did the fuel filter like 3k miles ago.

#12

Dont mean to hijack, but when i install the headers should i just buy new O2 sensors since im going to be in there anyways? When switching to headers does the JHM tune adapt to them with the 02 sensors or will i need to go in and get a reflash. If anyone has explanation as to why or why not it would probably make for good info in this thread.

Thanks and again sorry to somewhat hijack.

#13

The JHM tune will adapt, no problem. I didn’t replace any sensors and had no problems there either.

#14

A few months ago i had a post cat CEL come on (i have no cats) and we reset with VAG. Did a week later. Then it went away on its own and hasnt come back.

My fear is that the sensors (at least one) is on the way out so i think im going to just change them just in case when i do the swap. Thought?

#15

Do you remember the CEL number? I had a bad sensor and was throwing 2 codes.

#16

I dont. My buddy ran it at a gas station on the way down to h2o. Thats all i remember him saying. Ill see if i cant get it from him.