Our house is 5 years old. In November we noticed an odor, which I charactized as sewer odor from the toilet bowl. The plumbing contractor came out and check under the house for a sewer leak and there was no leak. We removed the toilet and replaced it with a new toilet. The odor is still there. We are at a loss as to what to do. What can be
Causing the problem?
Thanks

Hi Hook, Any relation to Captain Hook?

"The plumbing contractor came out and check under the house for a sewer leak and there was no leak. We removed the toilet and replaced it with a new toilet. The odor is still there"

So we know it wasn't the toilet or a leak under the bowl.
So now we look else where. What's left? Vent or partial clog sending a bubble of sewer gas back through the toilet bowl? Not likely since I don't see a complaint about a "gurgle",(blocked vent) or a "bubble" ( partial clog) coming back out of the bowl.
Have you taken the lid off the tank and sniffed around in there? Could be bacteria growing in there.
Tell you what let's do. Tonight before bedtime dump a jug of bleach in the tank. Soak a rag in it and scrub down the inside of the lid and the parts of the tank not covered by water. Let it set overnight and use the other bathroom. Next morning flush and sniff. Let me know if it helps. Cheers, Tom