So basically what the OFT does is tune the ECU so you don't throw any codes? Are all dealers going to go into your ECU to see if you've installed a tune? Since the OFT is a tune and not a piggyback, there's no physical evidence that you've tuned it. They'd have to pull the ECU data to see it from my understanding.

Generally any tune for catless headers will adjust AFR ignition timing valve timing ect for best horsepower from headers and it will also disable the CEL code for Cat Efficiency which is the one you get when removing cat.

With OFT you can flash back to stock but you will probably then get the CEL fro catless headers, the service guys are going to notice the headers anyway so little point in flashing back to stock.


If no tune use O2 sensor spacer on second O2 sensor. The first O2 sensor is used for AFR sensing for engine so its not effected (ie one before the CAT in header.

The 02 sensor spacers are put on the secondary O2 sensor to "fool" it to thinking the cat is still their, sometimes you will still get the odd CEL thrown.

Yeah, no.

The thing is once you have something like Engine Failure they are going to do is look into it deeply to place blame on you. So they'll check the ECU to see if it's been flashed at all. That and they'll probably look if there is any documentation of mods when you bring it in for service that the dealership noticed if they decided to do that... This is the only reason i refuse to tune this car because i want to enjoy having a warranty just in case some shit happens... after warranty runs out:burnrubber:

Your response is very descriptive thanks

@EpicNameBro : How different is the secondary cat in terms of functionality? I'm guessing relocating the sensors around the secondary is better than having both sensors reading exhaust gas coming straight from the engine, no? I'd prefer to wait until warranty expires before tuning.

I've been running my uel header for a full tank of gas now and I've had slightly better mpg with no cel yet.

They reduce emissions during the start up phase before the primary gets to temperature... So with that being said the secondary cat has no role whatsoever in O2 sensor function or emission control feedback function. The primary determines if the car is running too lean or rich and adjust as such. Going off the second cat gives a false reading which depending on your set up may be doing more harm than good...

From what I read, it does assist in catalyzing pollutants while the primary heats up. Once the primary is fully functional, the secondary has little to do with pollutants since the primary already had it's pass at the gas (pass gas lol).

My thought is that the secondary is still a catalytic converter, as long as it is converting/catalyzing within the acceptable threshold, it should be fine--aside from cold start emissions. I will continue to look for more info on this.

If you can point me to some reading on this I'd appreciate it. I read your first line on a BMW board.

This kinda makes me reconsider getting an openflash header... Is it really confusing as all that sounds or is there a simple solution?

There are simple solutions, but there are trade offs.

If you have emissions testing, You can do some research and try what some of these people have said, or just switch back to your stock catted header/stock tune every year or two. Will cost you $20 in gaskets, and a few hours in labor.

If you have a serious failure, and take the car in for warranty work, you may be screwed out of the warranty coverage because they can blame just about any mechanical failure on a tune, regardless of it's actual impact. In most cases if you are curtious and persistant you will still get covered, but this is just something you have to consider.

kind of makes me long for the old days when you didn't have to tune the car to accept a new air filter.

my ofh is scheduled to get here Thrusday, because we don't have emissions testing here.

I'm thinking of having a shop make me a new front pipe with a cat and place bungs before and after it for o2 sensors.

I really want to know the technical differences between the primary and secondary cats tho. If they are close to the same thing Idk if it's worth going through the hassle of getting a replacement primary.

If you had to replace the secondary cat, do exhaust shops sell 'secondary' cats or are they just slapping on a universal?

The primary cat people are referring to the one in the header. The secondary cat refers to the one in the front pipe.

....I know it's in the header. I have an aftermarket header and thinking about having a new primary cat installed elsewhere.

The secondary is mainly there to clear up whatever the primary didn't get in terms of breaking down gases etc. It's not there to monitor anything it just acts as a second filter once the car is warmed up. Essentially creating a fail safe for a car to meet the strict emission laws...

Yes, that is understood. But I wonder what differentiates a secondary cat from a primary cat technically/internally? Is it different catalysts? Less catalysts?

If you call an exhaust shop to replace a secondary cat due to failure or whatever, do they just slap on a universal regular cat in its place, or do they have special cats used only in the secondary position?

My goal in this is to determine a functional fix to losing the primary cat that was in the factory header. My possible solutions are:

1. Use the oem secondary cat as the primary, relocating the o2 sensors before and after it.
2. Have the oem secondary cat replaced with a 'primary' cat at an exhaust shop, and have the o2 sensors relocated before and after it.

This question might be best answered by a shop that specializes in exhaust systems (performance shop, not Meineke\Midas etc).