[23sport]

I have a Yamaha multi function digital gauge on Yamaha F225 that powers up when the key is turned on. Everything on the gauge works except for the tech, it reads zero when the key is on but will not show the RPMs when the motor is running, just stays on zero. How can I test the gauge to see if it is bad or if there is some other problem. A marine service center that looked into the problem said there was power getting to the gauge and they thought the gauge was bad, but they had no way to test it to see if it was for sure or not and then offered to sell me a new gauge. I don't really want to pay $300 or more for a new gauge.

[rodbolt]

look at the back, make sure the green tach send leads are still plugged together.
set your voltmeter on the AC scale and check for a tach send signal with the engine running. voltage will increase and decrease with RPM.