We have a power vent water heater that only lights intermittently. Have to turn unit off and on several times before it will light. Have read trouble shooting manual which mentions that it could be due to low gas pressure. Have had two different plumbers check unit and are unable to help. They have not checked for low gas pressure. Is this because this is not what they do or is it unlikely that low gas pressure is the cause. Unit is only 2 years old and we have had this problem for some time. Other reasons in the manual were low voltage, electric polarity to unit incorrect, hot surface ignitor not positioned correctly. Not sure where to go from here.

: Look out near the gas meter. You should see a inverted tee, (tee turned upside down)with a pipe pointing down. This is a drip loop installed to collect moisture so that it won't get in your gas line. Over time this drip loop will fill up and needs to be drained. If it's not drained water will enter with the gas and put out the pilot light. Some drip loops have a petcock at the bottom so you can drain it. Others have a hard cap that must be remover with a wrench. Shut off the gas at the meter, open up the drip loop and drain it. If it has a hard cap, don't forget to wrap the threads with teflon tape before you put it back. When you turn the gas back on paint the threads with liquid soap and check for leaks,(bubbles). Hope this helps and thank you for rating my reply. TOM
Answer : Let me paste up a article I found on the web that you may find of interest.
Troubleshooting Gas Water Heaters
Knowing how to light the pilot is one key to living with a gas water heater; see the instructions on the tank. For safety, a gas heater has a thermocouple. This is thermoelectric device that impinges on the pilot flame and shuts off the gas if the pilot light goes out. The gas flame should be blue. If it's orange, adjust the shutter; if it's still orange, call for service.
Twice a year, inspect the flue assembly to be sure it's properly aligned and all its joints are sealed. Then check the flue by placing your hand near the draft diverter (with the burner on); air flowing out indicates an obstruction that should be removed. Every year or two, shut off the gas, remove the access panel, and clean the burner ports, using stiff wire or a needle. If you ever smell gas, get out of the house immediately and call the gas company.
There is no hot water.<<
Check for:
Unlit pilot light.
Pilot light won't keep burning.<<
Clogged burner
Defective thermocouple.
The gas is not coming out.<<
Remedies:
Carefully relight the pilot.
Make sure the gas controls are completely turned on. Then check the thermocouple and be sure it is firmly connected to the gas control unit and positioned near the pilot flame.
Call a qualified contractor.
Replace the thermocouple.
Call the Gas Company.
There isn't enough hot water.<<
Check for:
Incorrectly set thermostat.
Defective thermostat.
Exposed hot water pipes
Too small of a water tank
Clogged burner.
Sediment has formed in the tank.
Leaking hot water faucets.
Remedies:
Turn the thermostat higher.
Call a qualified contractor.
Insulate the hot water pipes.
Install a larger water tank.
Call a qualified contractor.
Turn off the gas and drain the tank.
Repair or replace the faucets.
The water heater is very noisy.<<
Check for:
Scale and sediments in the tank.
Remedy:
Turn off the gas and flush the tank.
The water is too hot.<<
Check for:
Wrong setting on thermostat.
Defective thermostat.
Blocked exhaust vent.
Remedies:
Reset the thermostat.
Call a qualified contractor.
Check the vent and clear it.
Water is leaking from the heater.<<
Check for:
Leak in the draincock.
Leaking safety valve.
Hole in the tank.
Leak in the plumbing connection.
Remedies:
Close it tightly or replace it.
Check the water temperature. If it's too hot, the thermostat may be broken. If the safety valve is defective, replace it.
Buy a new water heater.
Call a plumber. <<< Before you call a plumber flush,(not drain) your heater.

Very well written!

Water heater won't stay lit after the water gets hot. I changed the thermocouple and it still goes out.

This is really a question not a answer sorry but I need a answer to this question