When a gas furnace gas valve turns off, can it still leak and allow some gas to vent from the burner ports?

I can smell gas from the furnace after it has been off for several hours. I have a gas leak detector and I check and am sensing gas at the gas ports (tubes) long after the gas valve is off. Is this a problem?
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On 12/8/2016 9:14 PM, Earl Robertson wrote:

Yes, it is potentially a problem. If, for some reason the valve is leaking by it has the potential for fire or explosion. Get a pro to check it.
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On Thursday, December 8, 2016 at 8:14:04 PM UTC-6, Earl Robertson wrote:

Get it checked out IMMEDIATELY!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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In typed:

Yes, of course it is a problem. If you smell gas hours after you believe the valve has the gas shut off, you obviously have a gas leak. In my area, the thing to do in this situation would be to call the gas supply company. My local supplier is PSE&G, and they come out immediately for any reported smell of a possible gas leak. It is free. And, they find the source of the leak and they tell the customer what they find. Usually, they can fix the problem for a fee, or sometimes for free, depending on where the leak is and what is causing the leak. They also give the customer the option of fixing the problem on their own or having someone else fix the problem other than the gas supply company. Again, no charge if they don't do the work.
You wrote, "long after the gas valve is off", but you didn't mention which gas valve that is. Did you mean a main gas shutoff valve before the furnace, or did you mean the gas valve in the furnace? If you meant the gas valve in the furnace, try shutting off the main gas supply valve before the furnace and see if the gas smell goes away. If that eliminates the gas smell, then the gas valve in the furnace is the probably problem and is somehow leaking some gas when it is supposed to be shut off.
Here is a YouTube video that may also give you some ideas, but it is not exactly on point regarding your question:
https://www.youtube.com/watch?v=0zKl9wvNIRk

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On 12/9/2016 10:24 AM, TomR wrote:

Or not.
Mercaptan is added to gas so it has a detectable odor. Gas piping can have an odor for weeks after it is taken out of service. It is possible he just has residual odor. Since his gas detector is finding something he should get it checked.
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TomR posted for all of us...

His new nym will be kaboom...
--
Tekkie

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On 12/8/2016 9:14 PM, Earl Robertson wrote:

Sometime back, I had a similar problem. I had replace the thermostat with a new one which used electronic switching (triac). Apparently there was enough leakage current through the thermostat triac that didn't let the gas valve close completely. So there was a very small amount of gas leaking when the valve was off. I called Honeywell and told them of the problem. They seemed to be aware of this and immediately sent me a new thermostat which had relays inside instead of electronics to do the switching. The "leak" stopped immediately. I think most electronic thermostats today use relays to actually switch the gas valve. Actually, in most newer furnaces, the gas valve is not usually controlled directly be the thermostat, but instead by the electronic controls in the furnace itself. But even so, my newer heating system, the thermostat has relays.
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replying to Earl Robertson, Bill123 wrote: This is commonly seen by gas utility first responders. If you’re concerned about current leakage from the thermostat or control you can check for voltage at the valve. Seems unlikely. Probably the valves aren’t seating properly because of contaminants in the gas. Just replace the gas valve. One exception: in very old thermopile systems a weak thermopile output can cause partial opening without ignition
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