We have a home we bought a year ago and had a washer and dryer installed on the third floor for convenience (no lugging wash up three floors)... anyway, we have noticed a sewage smell coming from the washer in the last few months... what could cause it??

I would suspect trap for washer drain. If leaking or broke, it will not hold water in "U', the low part of the trap that keeps the smell blocked.

Is this smell coming from the washer or the washer stand pipe?

I can't tell... I think it is from the washer... the rubber hose that goes to the standpipe, however, does have room in it... maybe if I plugged it with something so the fit is tight, that could help

i can't tell...i think it is from the washer....the rubber hose that goes to the standpipe, however, does have room in it...maybe if i plugged it with something so the fit is tight, that could help

Before you plug anything up Pour a 1/2 gallon of bleach into your washer and let it set over night. You may have bacteria growth down in the pump sump.
Next morning cycle the washer to flush the bleach out. Smell better now?
Regards, Tom

Yes, I did the bleach thing and the odor disappeared, but as you said, I will leave it overnight and see what happens... You mean the pump in the washer is gunked up?

"You mean the pump in the washer is gunked up??"

The pump in both washers and dishwashers have a sump where the discharge that's left in the hose drains back into when the pump shuts off. This sump can turn sour or build up bacteria over time and must be cleaned. Regards, Tom