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[Diamond Black Geezer]

Last night I was getting a new flange welded onto the exhaust cat, and in passing conversation my welding mate mentioned a Celica that recently failed MOT with weird Lambda results. He suspected that by him MIG welding a replacement flange on the exhaust, whilst the lambda sensor was still attached this may have fried, or damaged the sensor in some way. Only at this point do we do a double-take and realise he's just done the tack welds on my exhaust - with the lambda still screwed in.

My gut says that there's no 'circuit' to actually fry it, it should probably be fine. If you actually stuck a cattle prod into the plug, then yeah, that would probably damage it in some way. But I know nothing about the things, really, so happily deferring to your greater knowledge and experience.

 

[Webby the Bear]

I'm only just starting out welding....

However quote unquote..... to avoid damaging sensitive electronics disconnect the neg batt cable

[Marks DTM Calib]

Disconnect the battery neg and no issues, the only things marginally at risk as the integrated circuits and even then, these days, its questionable.

Plus, if the earth connection for the welder is close to the point of arc then the 'antenna' is kept small and the RF emitted minimised.

[Diamond Black Geezer]

Sounds like all is good in in the hood (or should that be 'under the hood')

So basically so long as there isn't an actual circuit to be made, you're fine? Along the same principle of a bird sitting on a 10,000volt power cable - there's no earth, so no circuit, yeah?

[Marks DTM Calib]

Not exactly, with welding the voltage is low but, you create an Arc, this produces high power RF which can cause an issue (the early radio transmitters were spark gap generators and welding near satellite dishes can cause people to start hitting their LNB with a broom.....ask my neighbour!).

So by keeping the clamp close to the welding point the aerial size is kept smaller and the issue greatly reduced.

[Diamond Black Geezer]

Ahh, one sees! Thank you Mark

To be honest I thought RF meant radio Frequency... but couldn't quite fathom how that would have anything to do with welding.. As you quite rightly point out, of course, there's an electric arc, and hence why suppressor caps were invented for plug leads. Thanks for the info!

In this instance the clamp was next to the welding point, the lambda being at the complete other end of the pipe. So looks like I dodged a bullet 

[Marks DTM Calib]

To be honest, I struggle to see how it could damage an O2 sensor any way as its little more then a zirconia 'cell' and a heater coil, it would take some real effort to give it a headache

[Diamond Black Geezer]

I'll text my mate who did the job, as he was scratching his head what could have caused the sudden rise in lambda emissions (on the Celica) - the only thing to have changed being his welding the exhaust up. Sounds like he needs to look elsewhere for the issue.

I'm just glad that I didn't spend all evening converting a knackered rusty exhaust with a decent sensor, into a decent exhaust with a shiny flange - but a knackered sensor! 

[Kevin Wood]

If you're talking about the "Lambda" reading obtained from an MOT emissions tester, then that value can be very inaccurate if there is a leak in the exhaust, so welding up the exhaust would have made a big difference. Hydrocarbons (HC) and carbon monoxide(CO) are the best indications of what's going on when you have an emissions problem. The Lambda reading is calculated based on O₂ in the exhaust and so it will be inaccurate when certain problems exist (i.e. exhaust leak or a misfire).

I too can't really see how you can damage a lambda sensor by welding, although if the wiring's still attached to the ECU it could provide a path for RF to damage the input on the ECU, so probably good practice to disconnect the sensor while welding.

[Diamond Black Geezer]

The exhaust was three miles away from the car, so definitely no problems there 

I'll definitely bear that in mind, making sure the is no leak. I'm hoping tonight to get a good, chamfered finish on the 'male' cat end, as this is new metal, to fit nicely inside the flared end of the silencer section, if you follow.