When Exhaust button pressed, vent fans not working, but microwave running for 30 seconds.

Yes. Faulty door interlock switches.
Replace the door switches with the correct ones.

follow this steps and fix it. God bless you

1. Exhaust fan motorIf your exhaust fan motor is not running, there are a couple of things that you can check involving the motor to locate the problem. After removing the fan motor, you can manually attempt to turn the fan blades with your hand. If the fan motor is working properly, the blades should spin effortlessly. If this does not occur and the blades are taught, the motor will need to be replaced. Secondly, the fan motor itself can burn out due to time and usage. Using an ohm meter, the exhaust fan motor should read open for continuity. If it does not, then the fan motor has expired and will need to be replaced.
   
2. ThermostatAnother part that could cause the microwave exhaust fan to not work is a faulty thermostat. A thermostat gauges the air temperature, which activates the exhaust fan when required. If this part becomes flawed, the exhaust fan will not energize and the thermostat will need to be replaced.
   
3. Main control boardIf your exhaust fan is still not working and the microwave fan motor, wiring, and thermostat are operating accurately, the root cause could be with the main control board. Replace the main control board.
   

Read more: http://www.appliancepartspros.com/repair-help/microwave-repair-help.html#12#ixzz2gURd1VtM
http://www.appliancepartspros.com

follow this steps and fix iT. God bless you

1. Exhaust fan motorIf your exhaust fan motor is not running, there are a couple of things that you can check involving the motor to locate the problem. After removing the fan motor, you can manually attempt to turn the fan blades with your hand. If the fan motor is working properly, the blades should spin effortlessly. If this does not occur and the blades are taught, the motor will need to be replaced. Secondly, the fan motor itself can burn out due to time and usage. Using an ohm meter, the exhaust fan motor should read open for continuity. If it does not, then the fan motor has expired and will need to be replaced.
   
2. ThermostatAnother part that could cause the microwave exhaust fan to not work is a faulty thermostat. A thermostat gauges the air temperature, which activates the exhaust fan when required. If this part becomes flawed, the exhaust fan will not energize and the thermostat will need to be replaced.
   
3. Main control boardIf your exhaust fan is still not working and the microwave fan motor, wiring, and thermostat are operating accurately, the root cause could be with the main control board. Replace the main control board.
   

Read more: http://www.appliancepartspros.com/repair-help/microwave-repair-help.html#12#ixzz2gURd1VtM
http://www.appliancepartspros.com

Some interesting reading on page 8 of the owner's manual:

"Boilovers cause smoking and greasy spillovers that may ignite and spread if the vent fan is operating.To minimize automatic fan operation, use adequate sized cookware and use high heat on surface units only when necessary."

And on page 20:

"If the temperature becomes too hot around the microwave oven, the fan in the vent hood will automatically turn on at the LEVEL 2 setting to cool the oven. The fan will automatically turn off when the oven has cooled down sufficiently. This is an automated process and cannot be turned off. The fan may stay on for 30 minutes or more after the cooktop and microwave controls are turned off. The Vent Fan will only operate in 1~4 levels modes while the microwave is working."

"Using the Vent delay off button
1. Press the Delay Off button once to turn the fan off after 1 minute.
2. Press the Delay Off button twice to turn the fan off after 3 minutes.
3. Press the Delay Off button three times to turn the fan off after 5 minutes.
4. Press the Delay Off button fiour times to turn the fan off after 10 minutes.
5. Press the Delay Off button five times to turn the fan off after 30 minutes."

If for some reason you don't have your owner's manual, you can still download it free here.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer

It could be that the fan button is sticking or a contact has welded itself closed therefore its getting power all the time.

Whirlpool customer service is here and you can also enter your model number on that page to download your owner's manual, installation instructions, etc..

The installation instructions are what you need.

We're happy to help you and we'd appreciate your thoughtful rating of our answer.

We had this happen to us tonight. We unplugged the microwave oven for 15 seconds and then plugged it in, but it the fan was still running. We then unplugged it for 30 minutes and the fan worked properly when we plugged it in again.

Do you feel air flow? That's the key.

Most microwaves have the exhaust fan assembly at the back edge, not up front immediately behind the grille / vent.

Post back if you can give us more detail or have other questions.

It sounds like your controller may be failing. Or it could be an electrical line problem. Any other electrical itmes giving you trouble? Has anyone measured the voltage level of your power line? It should be around 110-125 volts AC. If it's significantly lower or higher, you may have a bad connection in your breaker panel or in the wiring leading to the home.

From looking at the owner's manual, it looks like the vent fan is only supposed to come on when the internal temperature of the microwave gets too high OR when you press the VENT FAN pad. Here are a couple of fan-related excerpts from the owner's manual, which can be found here: http://products.geappliances.com/ProdContent/Dispatcher?REQUEST=SKUOBJECTS&DOCUMENT=Use%20and%20Care%20Manual&SKU=JVM1840BD (or the better URL here: http://tinyurl.com/yn7r6g) Automatic Fan An automatic fan feature protects the microwave from too much heat rising from the cooktop below it. It automatically turns on if it senses too much heat. If you have turned the fan on, you may find that you cannot turn it off. The fan will automatically turn off when the internal parts are cool. It may stay on for 30 minutes or more after the cooktop and microwave controls are turned off. - Vent Fan The vent fan removes steam and other vapors from surface cooking. Press VENT FAN once for high fan speed, twice for medium fan speed, a third time for low faw speed, or a fourth time to turn the fan off. Press the VENT FAN BOOST pad for extra fan speed. - Hope this helps.