The "toxic fumes" are contained in the exhaust system, so there's nothing to worry about from inside the car!
I had two
packs go on my L7X engine - both rear bank, which is a manifold-off job to replace - I dunno if this unit uses the Sagem
packs too though? The AA bloke didn't carry
for the V6 unit on his truck though, so I had to be towed back to a garage to get it fixed.
The AA bloke was able to find out the problem though. He had an IR temperature probe, which basically shines a laser spot onto something, and then tells you the temperature of whatever you're shining the laser onto. Very clever bit of kit.
He started the engine up and waited a couple of minutes, then "shot" the laser at the top of the spark plugs - sure enough, one of them was showing a much lower temperature than the others, so we knew it was that
pack that was at fault.
The Toxic Fume light seems to be a bit of a "jack of all trades" too - my other half had a problem with her Clio, whereby an inept Halfords mechanic unclipped a wire, which then fell down onto the cam belt, which sawed through it. This cable controlled the purge canister, and this also caused the Toxic Fume light to come on. A couple of blobs of solder and some insulating tape later, all was well
If you do have "dodgy"
(and hell, even if you don't!) it can be a good idea to buy one and keep it in your glove box...just in case you do have a problem with them! IIRC they're not that expensive, and it could save you a lot of hassle if / when you break down.