I have a model 3E381E overhead heater. the pilot tries to lightbut there is no gas coming thru to ignite. there is sufficiant gas in place as there is a water heater directly below running off the same gas line with no troubles. all the gas cocks are turned on but pilot light will not ignite. as i said.. the electronic igniter has a spark and tries to ignite pilot but to no avail

look for a bad gas solenoid valve: Spark is working, no flame = no gas getting to the igniter. Do you have gas? Is the main valve open?

Call Bosch
Gas Products Technical Support: 800-642-3111

That model number is only shown at one website, and that model number is not shown at Bosch.
Open following link for Bosch:
http://www.boschwaterheater.com/BoschHotWatercomHome/GasProducts/tabid/331/Default.aspx
Search of Bosch for that model number shows no result.

Some water heaters do not have a pilot light. Instead they use hot surface igniter rod that glows red hot
The igniter rod ignites the gas after solenoid valve opens and gas flows to combustion chamber.
The igniter rod and heat sensor have to be operational, and have to be positioned correctly for ignition to take place.
If igniter rod is not working, or if heat sensor does not detect flame, then gas solenoid valve shuts down and water heater does not heat water.

try cleaning the thermocouple or even replacing it because if the thermocoulple doens't sense the pilot light then it won't open the gas valve.

On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the igniter. The igniter will glow and you will be able to see it if viewed thru the small inspection port. Once the igniter gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.

What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the igniter does not energize (bad control, bad igniter), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not light (dirty pilot), the burner does not light (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control).

That is not the fix ...although it works...the pilot light is dirty as you suggested...it really needs to be taken apart to be cleaned properly....the pilot should NOT fluctuate in size after any amount of time...if it does the pilot is dirty , it's windy , or the gas valve could be defective....the pilot should be large enough to envelop the thermocouple...outdoor units get spiders etc also that block the pilots etc.....the problem with your solution is when the pilot isn't of sufficient size it's supposed to go out...the reason being you could get "Delayed Ignition" if the pilot is too small to light the burner or as a result of being small...is too far away from the burner

Depending on the make and model of your heater, the first thing I would check for is gas pressure. If it is sufficient, check the pilot assembly for corrosion and/or a damaged pilot tube. Sometimes replacing the entire pilot assembly is a sure fix and also replaces the pilot generator that degrades over time. A new pilot assembly takes care of a couple of problems by improving gas flow and millivolts generated. Also, the orifice inside the pilot assembly could be corroded or clogged.

The yhernocoupler would snuff the pilot if bad. What ignites the overhead burners? and what regulates the overhead gas supply? Look into this. Too much gas will not light as easily as proper balance gas and air. Even more so in a tower type heater.. Turn the gas on low and the pilot should grab it. Good luck

the thermocouple not only keeps the pilot on but lets the gas valve know its okay to open on a call for heat, remove your thermocouple and clean it with a fine grit sandpaper and if this doesn't work replace it all together.

I have several of these heaters in my rent houses and love them. The most problem I have is not being able to ignite the pilot with the user control igniter. Remove the screws that hold the front cover in place along with the front cover. Hold the pilot control down and ignite with a lighter. Keep the pilot engaged until thermocouple is warm enough to keep pilot gas flowing. If it will light this way then you need to blow out the pilot orifice. It seems that every year I have to go in after the summer season when the heater is off and clean the pilot/orifice lines with high pressure air. I believe spiders/bugs/dust block pilot orifice when it is not in use. You can also tell that orifice is blocked when you don't get a good blue pilot flame with pressure behind it allowing the flame to submerse the thermocouple. If the the thermocouple is not getting enought heat it will turn pilot gas off for safety reasons.

1 check power connection properly.