So I finally finished up replacing the front cat. PITA!!!! but doable. You will need a nice arsonal of tools. According to the BMW e70 service manual, you have to remove the rear cat first to get to the front cat and to remove the rear cat you need to take out the entire exhaust. :hitanykey: I didn't do it this way.

Here are the step I used. If you have a lift it will definitely save you the headaches and muscle pain from moving around. I used 2 jack stand at the jack points. Remove both o2 sensors from the cat. I sprayed the nuts at the flange of the cat and let them soak a bit. With a good few whacks from a hammer and a good socket set, I was able to remove both nuts with no issues. Looks like the bolts are welded in place so there is no way to remove the cat even if you remove all the nuts. Cutting it out was my only option. Used a Dewalt low profile sawzall and a new bimetal blade. There no room for errors so I was careful not to ding or cut anything near. I cut as high as I could to shorting the pipe so I could pull it out from the engine easier. (wish that was true) After cutting I removed the 8 nuts holding the cat into place to the manifold. I was able to pull it out with some maneuvering. You have to wiggle and jiggle and move it around as needed. Not fun at all.

Here's all the crap that was moving around. You can see a nice huge that fell out. It was definitely on fire and melting at one point. I made sure to suck up all of the debris from the resonator flange. Used my wet/dry vac and stuffed a rag in the left exhaust tip to help create more suction. I was able to suck everything out. Shaking and whacking the resonator pipe with a soft rubber hammer helped make sure everything got sucked up

Now comes the fun part. I tried to insert the new cat through the top of the engine. There's not enough room. After figuring out that there's no way it's going to fit i like this, I took a break. I needed a bit more space to the left of me which is where the ABS module sits, so I decided to pull it out. It's held in place by 1 10mm bolt and some rubber fittings. I unscrewed the brake lines carefully with a wrench. I used a pry bar to help it loose from the rubber fittings. Finally I was able to pull it out. You will lose a lot of brake fluid, so I need to refill and bleed the ABS system out. Now there's enough room to insert the new cat through the top. I used a cloth to gently pull the brake lines out the way while I was inserting the new cat.

Now it's time to insert the new cat. I was able to push it in through the open space now that the ABS module has been removed. So I inserted it to the left of the AC hose right were the ABS box sits. Lots of maneuvering is needed to get it to slip through. The new cat had some sort of bracket welded to it close to the flange. I cut it off, there's no need for it. It would have made it impossible to get it in with it attached. The aluminum sleeve on the AC hose took a beating. (oh well lol) I hate how its always in the way of everything. Finally got the cat in and ready to test fit. I'm beat and my body hurts lmao.

Man that looks brutal...


One comment...you MAY have a tough time bleeding that ABS unit. Getting every little bit of air out of all the circuits can be a bear. You need to actuate the ABS pump AND flush vigorously and repeatedly to get little bubbles out. I had an incident w my M5 (blew out a brakeline, drove 150 miles home and emptied the system) where I couldnt get a decent pedal feel. Finally gave it to a buddy at the dealer- took him 3 goes.


You cat sure looked bad. Gotta imagine something was going wrong for quite some time. Hope the new one does the trick

I'm going to run ITSA or INPA to activate the ABS module and bleed all 4 corners, hoping it will do the job.

Nice work. I bet the thing is running well after all the recent work.

Where did you get the replacement cat(s)? Dealer? ECS Tuning? There's a place in TX that sells replacement assemblies for less than $500 ea. Not sure if anyone has used them; but gotta be better than $1200+ each for OEM remans... Especially on a $7-8000 car...:dunno: https://www.discountconverter.com/di...&CylSelected=6

I got it off a eBay seller I've purchased from in the past. Good quality. Looks like it was made in Canada and welded together in the US.