Can the heater portion of a ~60's model 9090N be shut off or disconnected so that only the vent and light are used?

Fastest way to solve water heater problem is call plumber or service technician.

Open following link for typical error codes for intellivent gas valve found on newer Richmond/Rhemm power-vent gas water heaters:
http://waterheatertimer.org/pdf/Intelli-vent-gas-valve2.pdf
http://waterheatertimer.org/pdf/Intellivent-service-manual.pdf

There is no 100% sure way for novice to repair gas water heater.
This is because electrical parts have to ohm-tested using information from service manual that is typically not sent with new water heater.
Also parts have to be cleaned. The gas pressure checked, air in gas line removed, burner tubes and pilot tube and combustion chamber cleaned, burner parts inspected for problems, air intake cleaned, exhaust vent tested.
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

Best way for do-it-yourselfer to solve water heater problems is to learn as much as possible about specific model by reading product manual that comes with new heater.
The manual will show error codes.
Then keep up with yearly service recommendations shown in manual.

You're probably in for a service call and/or possibly water heater replacement if unit is more that 5-6 years old.
Your water heater plugs into outlet. Check outlet with cheap circuit analyzer to make sure the unit is receiving correct polarity without line interference.
If unit is in the attic, then add more venting to soffit and roof so pilot is not being snuffed out by hot air. If unit is in basement, then open some doors and a window and see if unit needs more air.
Most gas water heaters require yearly cleaning and service.
Clean the combustion parts and tubes.
Clean the air intake.
Have gas company check gas pressure.
Open following link for possible illustrations:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

The problem can be bad gas control valve, air in gas line, low gas pressure, gas shut off handle not fully parallel with gas line, dirty combustion parts, lack of combustion air, improper venting. Or combination of all things. It's an electronic gizmo, anything can be bad or shorted, and repair may not last long.

Without a trained service technician to ohm-test the parts using correct service manual, then we are guessing at the cause.

Power vent water heater implies that you have side wall venting which must be replaced with another side-wall vented unit -or- it implies rapid reheating of water.

If water heater is more than 6 years old, you might be money ahead by installing a new one from local box store. Avoid the power vent and electronics. Local box store will have passive roof-venting type water heater BUT box store does not sell side-wall power vent. If your power vent goes up through the roof, then box-store variety gas water heater will work. Total cost $350 do-it-yourself for 40 gallon gas.
Or get 40 gallon 240Volt electric for $220 and connect to circuit breaker box. Requires no venting, and is easy to repair for years to come.
Or small 120Volt electric that plugs into outlet to tide you over until gas repairs can be made.
Look for scratch-n-dent models and always offer 10% off.
http://waterheatertimer.org/How-to-install-gas-water-heater.html
Shower uses 5-9 gallons hot water.

If you have a leaking water heater there is a serious problem.
http://waterheatertimer.org/Water-heater-leaking.html
Page shows how to limp along on a leaking water heater if your water heater needs replacement.

One of three things is causing the water:
1) Loose water connection on top of tank that is dripping down inside tank >> check pipes on top of tank.

2) The vent is clogged or not working correctly causing condensation inside flue pipe.
Condensation water is dripping back down inside tank.
Problem means possible CO poisoning if gas is not venting.
Look at vent pipe to see if joints are corroded.
Use long fireplace match > light match and put into draft hood at top of tank and see if smoke goes up.
Put hand at draft hood opening when water heater turns on to see if vent gas enters home.
Do same test when furnace turns on to see if furnace vent is pushing back into water heater.
Does water heater vent inside same pipe as furnace? You may need to set up separate vent for water heater if furnace gasses are pushing back into home.
Check furnace vent for obstructions.
Look at vent stack top of home to see if hood is in place.
Check if bird nest is built inside warm vent pipe.
Improper flue is top priority for getting fixed so vent gasses properly escape home.
You may have to call a plumber.
Professional plumbing forum:
http://www.thetankatwaterheaterrescue.com/forums/forum3/

3) Your water heater is rusted out.
A rusted tank has to be replaced.
It cannot be repaired.
However, before replacing tank, you do not want a tank failure where tank ruptures and water runs inside home until water is shut off.
This will cause major damage.
Protect yourself first.
Turn off water valve above tank.
Move gas knob to 'pilot' so burner is not heating water.
Open tub spout on hot side to relieve pressure on tank so it will not rupture.

Now you're protected.
Link above shows how to limp along on failed tank until you can replace heater.

You need a new gas water heater from Lowes or Home Depot
Look at label on side of you tank for gallons and btu's of your water heater.
Measure height to flue pipe so you get same height water heater, so pipes don't have to be changed.
You need a pipe wrench and teflon tape which will be wrapped on each pipe thread when pipes are reassembled.

Step1) Drain water heater using valve at bottom of tank.
http://waterheatertimer.org/Water-heater-will-not-drain.html

2) Turn off gas valve going into heater.
When gas valve is perpendicular to gas line, then gas is off.
Dissemble gas line to water heater.

3) Disconnect flue pipe and support flue pipe with wire so it doesn't not fall down. Flue pipe joints should be held together at each joint with 3 screws.

4) Turn off water. Shut off valve is on COLD side.
Disconnect water pipes.

5) Move old water heater out of way.

6) Set new water heater in place. Put water heater in drip pan.

7) Connect water pipes >> cold goes to side marked cold.
Hot to hot.
Buy new flexible water connector pipes at home center for easy installation of water pipes.

8) Connect gas line exactly as it was connected before.
Home center sells flexible gas line connector for easy installation of gas line.
To test for gas leak after installation, mix liquid soap with water and put around gas line joints to see if there are bubbles.
Natural gas has less pressure than water lines, so gas pipes are easy to seal with ordinary wrench.

9) install TP pipe that runs to floor. Use old one off old heater.

10) Fill tank completely with water until water is coming out of tub spout.

11) Turn on gas, and light pilot as shown in manual.
New gas heater sometimes have small water leak when first fired up, so ignore a bit of water.

12) Save water heater manual like it is gold, and read everything in manual.

Certain water gas water heater models feature electronic controls with a flammable vapor sensor installed on the bottom portion of the water heater unit. This sensor is designed to shut off your water heater system to prevent fires in case flammable vapors are present within the area. If your flammable vapor sensor caused your water heater to shut down, check the area immediately for leaking gas or other flammable products. Common household chemicals such as bleach can also trip the safety system as well as unsuccessful burner ignition attempts. I'm not sure what control your unit has, but on a White Rodgers here's how it goes.


White Rodgers Intelli-Vent PowerVent Control Equipped Models

1. Step 1 Turn the water heater's blower switch off and disconnect the power cord for the water heater (refer to your particular model's operation guide for the exact location of the blower switch). Disconnect the Molex connector attached to the bottom of the control pad (the left most connector with five pins).
   
2. Step 2 Turn on your multi-meter and set it to read resistance or "ohms". Connect one of the multi-meter's probes to the number two pin and the other probe to the number three pin of the Molex connector (middle black wires). If the reading on your multi-meter reads below 9k or above 45k ohms, the flammable vapor sensor is faulty and in need of replacement; you cannot reset the system until this issue is resolved.
   
3. Step 3 Reconnect the Molex connector to the bottom of the control pad, reconnect the water heater's power if and turn the blower switch to the "ON" position. Press both the left and right buttons on the control pad at the same time. Wait for the green light on the control pad to blink and press the following buttons in sequence: Left, Right, Left, Left, Right, Left, Right, Right (you will need to perform these actions within 30 seconds of reconnecting the power cord to the water heater in order to be successful

Certain Rheem water gas water heater models feature electronic controls with a flammable vapor sensor installed on the bottom portion of the water heater unit. This sensor is designed to shut off your water heater system to prevent fires in case flammable vapors are present within the area. If your flammable vapor sensor caused your water heater to shut down, check the area immediately for leaking gas or other flammable products. Common household chemicals such as bleach can also trip the safety system as well as unsuccessful burner ignition attempts.


White Rodgers Intelli-Vent PowerVent Control Equipped Models

1. Step 1 Turn the water heater's blower switch off and disconnect the power cord for the water heater (refer to your particular model's operation guide for the exact location of the blower switch). Disconnect the Molex connector attached to the bottom of the control pad (the left most connector with five pins).
   
2. Step 2 Turn on your multi-meter and set it to read resistance or "ohms". Connect one of the multi-meter's probes to the number two pin and the other probe to the number three pin of the Molex connector (middle black wires). If the reading on your multi-meter reads below 9k or above 45k ohms, the flammable vapor sensor is faulty and in need of replacement; you cannot reset the system until this issue is resolved.
   
3. Step 3 Reconnect the Molex connector to the bottom of the control pad, reconnect the water heater's power if and turn the blower switch to the "ON" position. Press both the left and right buttons on the control pad at the same time. Wait for the green light on the control pad to blink and press the following buttons in sequence: Left, Right, Left, Left, Right, Left, Right, Right (you will need to perform these actions within 30 seconds of reconnecting the power cord to the water heater in order to be successful

Certain Rheem water gas water heater models feature electronic controls with a flammable vapor sensor installed on the bottom portion of the water heater unit. This sensor is designed to shut off your water heater system to prevent fires in case flammable vapors are present within the area. If your flammable vapor sensor caused your water heater to shut down, check the area immediately for leaking gas or other flammable products. Common household chemicals such as bleach can also trip the safety system as well as unsuccessful burner ignition attempts.


White Rodgers Intelli-Vent PowerVent Control Equipped Models
1. Step 1 Turn the water heater's blower switch off and disconnect the power cord for the water heater (refer to your particular model's operation guide for the exact location of the blower switch). Disconnect the Molex connector attached to the bottom of the control pad (the left most connector with five pins).
   
2. Step 2 Turn on your multi-meter and set it to read resistance or "ohms". Connect one of the multi-meter's probes to the number two pin and the other probe to the number three pin of the Molex connector (middle black wires). If the reading on your multi-meter reads below 9k or above 45k ohms, the flammable vapor sensor is faulty and in need of replacement; you cannot reset the system until this issue is resolved.
   
3. Step 3 Reconnect the Molex connector to the bottom of the control pad, reconnect the water heater's power if and turn the blower switch to the "ON" position. Press both the left and right buttons on the control pad at the same time. Wait for the green light on the control pad to blink and press the following buttons in sequence: Left, Right, Left, Left, Right, Left, Right, Right (you will need to perform these actions within 30 seconds of reconnecting the power cord to the water heater in order to be successful).
   

Either the high limit switch is either defective or the wire has come off and the circuit is open. It is found under the heat shield on the front of the heat exchanger. Or there is a blockage in the flue or the blower motor is not running. Take the to trim piece off that is closest to the wall and disconnect the gray plastic hose off of the vent termination and try to start the unit. If it starts, you need to clear out the outer ring on the vent termination. If it continues to fail, there is still a problem and now you will need the installer back. Check all electrical connections going to the circuit board while you have the front cover off.

The problem you describe is usually caused by a thermal fuse intended to shut the dryer motor down and keep it off because of a problem with air flow within the dryer.

The following process will allow you to test the thermal fuse and replace it if you find it open.

Unplug unit and turn off gas if gas model. Check to be sure you have 240 Volts at wall outlet.

Most parts likely to fail on electric dryers are inside the rear cover. Heater is on right inside can, safety thermostat on lower left of can, High limit thermostat (non-resettable) higher left of can. On the blower housing to the left are the operating thermostat and, in some models, a thermal fuse (white plastic). ALL can be continuity checked easily. Always disconnect wire on one side of what you are testing. One will likely be open. Replace the component that fails the continuity test.

Replacing the heater, if necessary, can be done by disconnecting the wires at the lower left of the heater can on right. There will be a screw on each side edge of the lower half of the can. Remove the two screws and pull off the back lower half of the heater can and reverse the above process to install the new heater.

The thermal fuse or high limit thermostat failure generally points to a reduction of air flow due to SOME blockage either inside the dryer or in the vent pipe leading out of the house. If you find either of these parts bad be sure to check the vents after replacing parts to ensure the same parts won't fail again.

the valve handle is on when the handle is in line with valve body, off when crossed...are you pushing in on the pilot gas valve under unit? if not.. open bottom panel.. turn knob to pilot..push and hold down while sparking..if pilot lights continue to hold for about 60 secounds.. then release knob and turn to on! Also FYI... these units have a vent recall on them! please go to google and type in your heater, then model #

Disconnect the plug. Disconnect the wires to the heater element. Using an ohm meter or test light,test the heater element to see if it is open. If it is open the meter needle will not move If a test light , it will not light.  If true then replace element.