Suspected CAT problems - Harry
My car has been giving off horrid egg like smell for a while now, I have a feeling the CAT isn't working properly. Also the car sometimes shudders and then cuts out when I release the accelerator. What do you guys think:-

Are the smells coming from a faulty CAT and is that related to the car struggling to stay on at times?
Suspected CAT problems - Andrew-T
An 'egg' smell means hydrogen sulphide (if you have identified it correctly) which itself means sulphur in the fuel. A cat usually only smells of egg before it has got properly warmed up. After that it should oxidise everything fully and give sulphur dioxide instead.
Suspected CAT problems - Harry
Thanks for your suggestion, but do you have any ideas for why the car is shuddering then cutting out? Also when you say I have sulphur in the fuel what does this mean for the car, bad/good? And if it is bad what can be done about it.

Suspected CAT problems - Andrew-T
All petroleum contains some sulphurous components, but the proportion varies a good deal. Refineries 'sweeten' their product to reduce sulphur to a tolerable level, and most diesel is now Ultra-low Sulphur, as all sulphur combustion products are bad news. You could try a different brand of fuel to see if anything changes. But I doubt very much if it has anything to do with your engine's running problems. Better give more details about the car and hope one of the experts responds.
Suspected CAT problems - RichardW
The funny smell is often found with catalysed cars before the cat warms up. This suggests that yours is not working. I wonder if it has broken up and is partially blocking the exhaust - which could also account for the shuddering etc. A quick check with an exhaust gas analyser should show if the cat is working or not.

Suspected CAT problems - Omegaman
This could be several things. as other contributors mention, CATS are not too efficient when the car is cold and the precious metals inside the CAT take a while to warm up and do their job of acting as oxidising agents. In my limited experience newer cars seem to be particularly prone to do this when cold (don't know why?). If the car's hot and you smell hydrogen sulphide ("rotten eggs") in strong quantities then it's indicative of the car running too rich for some reason ie there's too much fuel being burned for the CAT to efficiently oxidise everything coming through the exhaust manifold - or the CATs on it's way out. Sounds as though you need it checking out - any warning lights "on" on the dashboard (eg engine management light?). How many miles have you done/what's the age of the car?
You might find petrol that is relatively low in sulphur but as far as I'm aware there's not currently a brand on the market that is totally free of sulphur. Besides - hydrogen sulphide can be smelt in miniscule quantites.
Suspected CAT problems - Harry
Thanks for all your replies, Omegaman you were asking whether there is an engine management light coming on. I should have mentioned this factor because it does come on for about a minute or so after driving on a dual carriage way around 60mph. My cars done around 80000 miles its' a Peugeot 306XSI and the smell occurs mostly when the car is warm. Do you have any idea of how much it would cost to get the CAT checked/replaced?
Suspected CAT problems - Omegaman
Engine management light coming on is indicative of a fault that can't be ignored - vital piece of evidence this.
Suggest you get the CAT tested - any MOT station should do this for you - £15 to £20 would be fair for this alone, this would also check out the Lambda sensor (oxygen sensor on the downpipe before the CAT - this could be faulty causing the car to run rough). New CAT probably £250-£400 fitted, I'm not well up on Peugeot prices.
If your CAT's in order you'll need to get somebody to check your engine management for fault codes - need a computer reader for this, best done at a good independant garage or a main Peugeot dealer. Engine management system will store them until they're cleared.
Good luck.
Suspected CAT problems - Harry
I had my MOT done about a month ago and I can remember clearly the mechanic telling me the car passed its emissions test which involves the whole exhaust system. I do feel I need to take the car to a Peugeot dealer to pin point the fault shown on the engine management system, do you have any idea of how much this costs?
Suspected CAT problems - Omegaman
Shouldn't be more than an hours labour - in reality they should have an idea whats wrong within 5 minutes of plugging in their computer ready. Pug dealer charges are about £40 an hour I would guess, a good Independant garage could save you £10-£15. Main dealer probably your best option though.
Suspected CAT problems - Dizzy {P}

I used to run a Citroen ZX 1.4i. After pressure washing the engine, some water got trapped underneath the spark plug covers and caused misfiring for a few days when the engine was warm. This made the exhaust give off a rotten egg smell and the engine management light would also occasionally come on. Once I'd got rid of the water, the engine ran perfectly and the next MoT test showed the catalyst to be fine.

What I'm saying is, the symptoms that you are finding may be due to a simple fault and the catalyst is probably fine.

Value my car