How to fix your gas furnice

Even though you are smelling gas it could mean the gas flow is not strong enough.
When you say the water flow seems good, have you cleaned your filter recently?

If you are somewhat handy you can troubleshoot this heater by taking off the front panel turning it on and looking up to the back of the circuit board.
When it shuts off it will have red lights next to abbreviated words such as hl1 hl2.

Whichever light or lights are red will point you in the right direction of the cause.

Is this a stand alone water or is it run by a control panel?

If you have a control panel that turns it on then make sure the temp reading for the heater display and the other control panel are the same or within 1 degree of each other.

If it a stand alone heater woh. Onautomation controlling it, you would want to check the actual water temp of the water to see if it matches the heater readout.

If it does then you want to heat it back to 100 , once the service light comes back on, look at the back of the circuit board, most likely you will see a hL-1 or hL-2 with a red lobby next to it.

These are high limit sensors....if one of these has a red light next to it then you need to ensure proper water flow , possibly clean your filter and ensure flow.

If it persists then the issue could be internal heater parts, thermal regulator, bad bypass, failing high limit sensor.

Most gas water heater problems today can be solved with yearly maintenance that includes cleaning burner parts, clean air intake, check venting, check electrical connections, bleed gas line, and check gas pressure.
Gas water heater manuals recommend yearly maintenance by service technician.

In past years, gas water heaters required less maintenance, but they were less energy efficient and more dangerous to operate.

Sounds like you have Intellivent gas control valve thermostat.
Open following link to identify gas valve and see error codes:
http://waterheatertimer.org/pdf/Intelli-vent-gas-valve2.pdf
http://waterheatertimer.org/How-to-adjust-temperature-on-water-heater.html#intelli

Error light on vacation + 2 other lights = 3 possible conditions:
1) Wiring error: check outlet with circuit analyzer:
http://waterheatertimer.org/images/Typical-circuit-analyzer-15.jpg
2) Pressure switch tubing not connected, air intake obstructed (clean intake), or vent exhaust obstructed (check for proper venting).
http://waterheatertimer.org/images/Test-water-heater-vent-400.jpg
3) System in lockout: Low gas pressure, hot surface igniter not positioned correctly.

You can do some of this yourself.
http://waterheatertimer.org/How-to-install-gas-water-heater.html#clean

Solid red typically means bad controller. Try unplugging the furnace, wait about 30 secs, then plug it back in. The possibility that the controller was sending voltage to the valve even when there was no flame is a bad sign. If the controller starts the furnace, watch it very closely and see what it does. If the solid light comes back on, it's time for a new board.

On the bottom door where the blower foor is located,
look on the back of it. It should have a Electrical print
and at the very bottom A Troubleshooting chart for
the control board. It is hard to find but it is on there.

Usually a solid Red means system is ready and good
to go.

Good luck, if you need anymore help let me know.