sewer odor coming from a/c? Pls advise

First thing I would do is try to decide if the smell is coming from the a/c or the drain. If you don't have a good filter on the a/c or you don't clean it often enough, dust can get onto the evaporator coils. The moisture from the humid air in the house and the organic dust/pollen/etc. on those coils can grow some nasty things that can smell. In the winter, it all dries out and the smell goes away. Getting a good look at the coils can be a pain, depending on the system. If the smell is only from certain ducts, then it is possible a mouse or something like that crawled up there and died, too. If it was the coils on the a/c, you'd notice it everywhere there is an outlet. If the bathroom isn't used often, it is also possible that the trap has dried out. Try pouring a quart of water down the drain. That may solve the problem.

continue on sewer odor.

To jadnashua
Thanks for your response, it seems very strong at the drain, the bathroom i don't use oftently, since we move in the house last September.
so your suggestion about If the bathroom isn't used often, it is also possible that the trap has dried out. Try pouring a quart of water down the drain. That may solve the problem. is possibly right.
Is there anyway that I can see the trap somewhere? Is that normal problem if the bathroom don't use and cause the sewer odor? Is there something that i can prevent this problem.
Thank you in advance

sewer odor

I ran the water, and it seems the odor went away, but is it normal problem for most of house, or do I need to resolve something on the trap, in order to prevent this odor coming from the drain in the future.
Regards,

Water in the trap is what keeps the smells out of the house. If you don't run water through the drain often enough, just like any water, it evaporates. Then, you have an open path to the rest of the sewer. So, maybe once a month or so, run some water down that drain. This depends a lot on how dry it is...if you live in the desert, it will happen much more often than if you live in Florida for example.

One thing we didn't ask yet about the odor: is it really sewer gas odor, or another smell. I realize this is tough to diagnose. The other 2 odors I am thinking of are (1) mold or mildew from the evaporator coil being carried down the condensate drain. and (2) "gunk" odor from decaying hair etc. right in the pop-up drain, above the trap. This can be cleaned mechanically and with disinfectants like Lysol.

drain

Your A/C condensate drain is probably piped to that sink's' drain so when the trap dries out because of lack of use, the "negative pressure"/ vacuum created by the blower will draw the sewer gases into the A/C unit and then blow them through the house.

Sewer Odor

The odors are coming from your roof top plumbing vent pipes...because that's the way they were designed....charcoal filters can solve the problem

I had a problem with odor coming from the overflow in a bathroom lavatory whenever the water was run in the sink. I solved the problem by putting a dose of chlorine bleach directly into the overflow. After a couple of doses over a period of a week, the odor has not returned in about 3 months.