You should have or get or make a simple diode checker. A diode is really simple. If you put voltage across it with neg and pos one way you'll see electricty flowing through the diode. If you turn the connections around you won't see any electricty. Compare that with a light bulb. No matter if you put the wires one or the other way the bulb lights. So the only resort if neither try works for you is to check the diode that it's working. It's simple. You put a meter with one lead there and theothe lead over there. IF you see a signal then you swap the leads. This time you should not see a signal. It works one way. What woudl you think if you saw a signal no matter which way you put the meter leads? The diode would be broken. You would have to replace it. What would happen if no matter which way you put the meter leads and you got no reading or signal from your test? The diode would be broken. You would have to replace it. The reason you don't just replace it is this: It's poor logical thinking. It's better to prove that it's broken so that when you put in a new one and the new one does not work either.. then you know something else is ALSO wrong. So 1. Get a meter for $10 at Sears or Harbor Freight. It will be worth it for later. Learn to use it. It's acutally fun. 2. Check the diode. You can find online how to use your meter to test the diode. When you're at the shop where you buy the meter take your diode with you if you can. If you can't then try to buy one that is approximately the same physical size. AT least it will fit mechanically. For specifications they're simple. They are usually rated at the highest current. 1 amp is typical for this kind of application. IF you hve an old door bell with a ringer and buzzer then get a 2 or 3 amp if possible. If it's electronic than anything should be OK. But get 1 amp if possible. The other spec, the voltage should be 100 volts or more. They're all this or more so no worry. 3. Test the diode. Two ways to try it. One way, then the otehr. It shold be opposite readings each way. Test the good one, first, instead. It, if it's good, will show you how the diode works. The meter puts voltage into the diode. When the diode lets electricity through the meter then the meter reads a signal. If the diode blocks the signal from getting through the meter then the meter reads nothing, 0.00 . So you've got a whole 2 weeks of electronics training in 3 paragraphs and 10 minutes. Try the diode one way, if it works you're done. If not try it the other way. If it works. .. etc. If it does not work either way then A. Check the diode again to be sure it's still OK. If not then you have a different problem It's nearly 100% sure it will be OK. B. If it doesn't work and diode is OK then get sommme more advice from only or here. You might not be getting DC or AC voltage from the bell's transformer. We'll be guessing at that point. I'll check back to see how you're doing Andre

Diodes are meant to allow the flow of electricity in one direction and block it in the other it might be possible that if it was installed wrong that the doorbell got fried. Just a possibility

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One possibility is that your meter is reading "phantom voltage" and the transformer actually can't supply significant current. To find out, check the transformer's voltage under load (when the doorbell button is pushed). Or connect a car dome light or tail light bulb across the transformer's secondary (screw terminals) to see if it lights. If it still reads 12 volts, or the bulb lights, remove the transformer, doorbell, and button from the house. Wire them to one another on your workbench. If they work there, either the house's doorbell wiring is faulty, or a connection to one of the components wasn't properly made. Wait, you said the transformer is in the garage? Any chance that it's downstream from a GFI?