Re: Emission Control System (ECS) light, Rotten Eggs

["Scott M." <scottmo@teleport.com>]

Craig,

Sounds like the fuel mixture may be off and possibly the
Engine Control Unit (ECU) can no longer adjust the mixture
within its range of control, hence it lights up the ECS
indicator. Could be the O2 sensor(s) are defective or the
hot wire mass air flow sensor idle CO% adjustment is no
longer correct due to other problems in the engine (vacuum
leak, cracked breather hoses etc). The ECU can also detect
faults in the other engine fuel injection/ignition
components.

Regarding the rotten egg smell, this  is caused because
while running under lean conditions the sulfur found in
gasoline is converted  into sulfur trioxide during the
combustion process.

Then under relatively rich running conditions, idling or
full throttle acceleration the sulfur trioxide is converted
into hydrogen sulfide within the catalytic converter. It is
the hydrogen sulfide that has the rotten egg smell. Audi
issued a service bulletin explaining this condition.

Some brands of gasoline with higher sulfur content can make
this problem worse.

You should have the car checked out and have them see if
there are  any fault codes stored in the ECU. 

HTH
Scott M.