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Discussion Starter · #1 ·
Cat was changed three months ago and I want to change the rear sensor, so you'd think it wouldn't be that seized up. But I can't shift it with an adjustable wrench.

Any tips? Try when hot? Give it a squirt of release agent? Get proper combination spanner (my plug socket tool for the front won't fit on the rear).

Thanks in advance.
 

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Wife's 01 1.8T mechanic
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Here's my tip: don't change it. The rear sensor only tells the computer the cat is working. The front one is the one that is used for all the A/F calculations. Unless the rear on fails/throws a code, leave it and save your cash.
 

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I would purchase a 02 sensor socket from your local auto parts store; I had no problem, replacing mine (I do live in the desert; no major rust here). The 02 special socket; makes it doable and it wasn't hard. Make sure to get a quality oe sensor from Bosch: look up here with your vin and engine code for the correct one:

http://www.boschautoparts.com/tech/vehiclepartfinder/pages/vehiclepartfinder.aspx

02 sensor socket (slit machined in the side wall; for clearance of the wire/plug):

https://www.google.com/search?q=02+...:en-US:official&client=firefox-a&channel=fflb

Be careful; when installing the sensor, that way you don't damage your new part! :cool:
 

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O2 sensors rarely go out, it's usually the actual cat that's the problem (maybe the aftermarket one you installed isn't working to factory spec?). The only thing that sensor does is tell the computer the cat is working, absolutely nothing else.
 

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Discussion Starter · #5 · (Edited)
Thanks for the reply. I have a socket with cutaway but it's quite tight to get on. I think the garage took the old cat off, swapped the sensor over, then fitted the new cat. I might have another shot at it. Don't suppose getting it hot will make a blind bit of difference? Apart from burning me!
 

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Discussion Starter · #6 ·
I get a 16806 code, main catalyst below efficiency, or something like that. New cat, new front sensor, still does it so was going to replace rear one. Other stuff like MAF, plugs, air filter and hoses are good and car seems to run fine.

Got a bit of gunge in breather hose, which I read is typical, so getting garage to change thermostat.

Q: Are front and rear sensors interchangeable? (Albeit lead length may differ.)
 

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Discussion Starter · #8 ·
K, ta. Well I'll get the thermostat changed first and take it from there.

Oil's 0W/40 synthetic and it still gunges up, so guess engine's running too cold.
 

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Discussion Starter · #12 ·
Rear lamba replaced by garage - it looked all coked up - and thermostat replaced. Now heater output is too hot for your hand and car warms up really quickly. Previously the heat output was dismal.

Emission light is out and car goes better. So, very pleased!

Thanks for advice!

Oh, ESP light is on all the time now. Don't think I'll get involved with that, unless there's a fuse I can remove so it goes out!
 

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Discussion Starter · #13 ·
Spoke too soon. Emissions light on again, even though car passed all the emissions tests on MoT. Arrrgh!
 

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The catalyst below efficiency code can be caused by an exhaust leak in the section between the two sensors, so either at the gasket between the manifold and the pipes or in the flex sections. If your new converter was welded in it could be a leak at one of the weld joints. Do you have any signs of an exhaust leak?
 

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Discussion Starter · #15 · (Edited)
Not noticed especially, although it sounds a bit 'puffy' - thought that because it wasn't a VW one. Will have a look. Q: If the car has passed its MoT on emissions, can the coolant temp sensor or mixture still be too rich? Enough to flick the light on?
 
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