Removing Lambda sensors in situ

I need to replace all 4 of my Lambda sensors as 3 of them are logging fault codes and my fuel consumption has really dropped in traffic. I know the threads can corrode but has anyone managed to replace them without removing the CATs first?

22 mm and unscrew the oxygen sensor access and if they can actually come out without a fight is another question

Yes, but it will be far easier with the bumper and head shield removed. It's easier to undo them whilst stil in situ as they are held tight whilst you apply torque to the threads. Trying to undo them with the cats free on the bench is far more difficult.

Plumbers butane / propane mix blow lamp around the circular mounting boss will make the job much easier.

Quotes source post: 2707338

+1 to this. I had to get pretty medieval on the old Lambda sensors in my car. I addition to the heat, I also chopped the cables of to get better access with a socket.

Mine is a 1999 and only has 2 lambda sensors. They were on there pretty good, but came out in-situ with a bit of heat and a proper lambda socket.

Thanks all, I had a look at access and it’s not good from underneath, someone in the past has had a go at them with a 22mm spanner and some of the flats have started to round. Remove bumper & heat shields, soak with Plus Gas, leave CATs in position, Lambda socket or cut the wires and use a deep socket, lots of heat and copious amounts of swearing, have a tea break and repeat. Sounds like a fun job.

It is honestly nuts to try to remove them in situ if they are original. My car has been extensively garaged based on the minimal corrosion on the underside and I have never been able to remove a lambda sensor without cutting off the body and basically melting out the threaded section. Unless you live in California or they have been replaced recently that is the most likely option unless you're super lucky. It's also pretty trivial in the grand scheme of things to remove the car pipes as access is good.

I’ve ordered new exhaust clamps just in case the CATs need removing. I know what will happen though, it will take me a day to remove 2 or 3 of the sensors and I’ll damage the CATs removing the others requiring replacement and none use of the car for a week!

the bolts attaching the cats to the manifold also rust through but these can be cut off pretty easily. Then drill out and replace with stainless.

I started this job last week, ive got new cats to go (one has a big dent in the pipe near the clamp to the backbox!) i was hoping to reuse the sensors, but reading this thread is making me think 4 new ones are required

I had to remove parts of my exhaust to get my lambda off as a regular gas gun wouldn't shift them, I cut the top off to be able to get a normal socket over the top but it still wouldn't budge. I took it to a local garage who had an heat induction tool and it came off in seconds, they are a great bit of kit if you can justify the cost of a good one.

Update - as expected a frustrating day! Bumper removed but access still terrible so the CATs had to come off, I had replaced the exhaust 8 years ago and used Marine grade SS fixings which still put up a reasonable fight, I had to angle grind the joining sleeves apart also. Sensors rusty, Amazon sensor removal split socket set was useless so I cut the tops of the sensors and ground the nut down to 21mm to make it nice & clean. Loads off heat and a breaker bar and they started to turn… 2 sensors ripped their threads off & the other 2 snapped! Grrrrrr. The snapped ones are now cut flush and drilled out to 13mm. I need to buy reduced shank drill bits in 0.5mm steps up to 16.5mm and a M18 x 1.5 tap and hope the next stage goes well otherwise it’s going to get expensive if I need to buy new CATs. Oh and it drizzled most of the day despite the forecast saying it would be dry!

One other option would be to buy new sensor bosses and weld them in. Is it critical to get the sensor depth mm perfect? How would I cut out the old boss to get a nice clean hole? Even if I used the angle grinder to take the top off it would take ages to drill 4x 22mm holes through what is left and I’d need to buy loads more bits to step up to size. EDIT - stepped drill bit and a die grinder bit to enlarge seems the way to go, hopefully the drills & tap will work.

I have given up with trying to reuse mine, and looked at 4 new ones, the existing ones are already rusted enough for a 21mm spanner to fit, shame as they are working ok. Only managed to get one cat off too, i’m not ambidextrous enough to use a hammer well enough with my cack hand on the passenger side flange, resulting in the punch slipping off the flange bolt, spinning out of my hand and somehow rebounding into my chin . I’ve given up for this week with a plaster on my face