Modern Vespa

2017 Vespa GTS300ie. Randomly fails to start, but will eventually, sometimes hours later. Brake light switches work, replaced starter relay, checked connections and grounds. Fails with program key as well as daily key. Turn key on and pump presurizes fuel system. The coolant temp LED flashes twice, half a second apart, followed a second later with a single flash. I haven't been able to find out what that code means.

ah ... the coolant led? hmm my first thought is that's a problem with the sensor, possibly simply the connector needs unpluging and clean the contacts and replugging. (I'd expect if there is a problem with coolant sensor then the ecu would refuse to start the engine)

Do you mean the 'code LED'? The one that flashes every second or so when you turn the scooter off...

And what happens when you hit the start switch?

Replaced starter relay. Tilt switch is OK. Coolant temp sensor is OK. No sidestand switch. Wiring I can access seems sound. What happens when I press start. No relay click.

Some more info: I put in key. Turn to ON. Pump presurizes. Pull brake lever (checked both brake switches, both work) IMMO/WaterTemp red LED flashes for half a second, then off for a half a second, the on again for a half second, then off for a full second then stays on. All this time the EFI/ASR amber LED is flashing.

So, I disconnected the battery for 20 minutes. Upon reconnection it immediately started. Points me to the ECU.

My Vespa is a 2017 GTS300ie. Can you tell me where the computer is located. My Haynes manual isn't clear where the ECU is mounted, right side leg shield or attached to the throttle body. I want to take the ECU to the 'local' dealer, 100 miles away to have it checked. This is one of those dealers where the valve adjustment for a single cylinder engine 'starts' at $500. So, you can see I'm reluctant to set foot there.

Thanks for your help.

If that LED stays ON then the bike will NOT start. You have a 'one-flash' diagnostic code (the first 0.7 sec flash does not count).

It's unlikely to be the ECU.

https://www.scooterwest.com/so-ecu-throttle-body-for-vespa-2015-2017-euro-3-miu-gts-gtv-super-300-abs.html

The computer is part of the throttle body. They seldom go bad, but if they do, you're looking at right around $500 for a new one. The part number is CM084833. But before you throw this kind of money at it, and end up with two working CDIs that are both programmed to your master key, you would do well to take your bike to a dealer and let them plug it into the PADS computer.

Check the coil and the wire coming off the coil. If you gently tug on the spark plug wire and it comes out of the coil, that's likely the cause of your problem. Also, make sure the battery is correct and the positive terminal isn't close enough to the frame that it's arcing from time to time.

Thanks all, especially what the flashes mean. My local dealer, home of the '$500 to start valve adjustment' is a place I avoid if possoble, plus they are 80 miles away. Rent a truck situation. For the throttle body @ $700 in the response, my dealer wants +1,400. I'll order the antenna. Again, thanks a bunch.

My Vespa not starting tale. I believe that it is the computer. But maybe reading it will give you some pointers.
So, about a year ago I noted that Whitey, a 2016 Vespa that I purchased on ebay and rebuilt the top end, was having an intermittent start problem. I would press "start", and it would click, and then click about every 4 seconds. When it wouldn't start, sometimes just taking it off the center stand and retrying would encourage it to start. About 5 months ago, it reached the point that I couldn't trust it even to eventually start. So troubleshooting:
1) swapped battery with Blackie... no change
2) swapped relay... no change
3) purchased a used starter from AF1, $30.... no change
4) replaced start switch, $10... no change
5) replaced kill switch... no change
6)pulled, checked, and reseated all fuses , no change
7) bypass the relay. connected the starter cable to battery cable... cranked right up every time (notva problem with starter or battery.
At this point, I had replaced everything in the start circuit but the wiring harness and computer. So then I pulled out the VOM and the wiring diagram, and started checking things. According to the wiring diagram, the relay is energized when two things happen:
1. ) When brakes are pulled, the relay is connected to 12V (VOM verified okay)
2. ) When the starter switch is pressed, a signal is sent to the computer to drop the pin connected to the other side of the relay to near-ground. This would create a current in the relay, causing it to latch and connect starter to 12V and crank.
Unfortunately, the computer side of the relay does not go reliably to ground. This, again, means either the wiring harness or computer is misbehaving. There was no way I was going to replace either one of those when the bike will actually start and run, so I rewired the ground side of the relay to the start switch and ground. Press "start", ground relay and start bike. The bike still has to be on with the key to start, and the kill switch still kills it. It took me about 4 hours to rewire it, but today I rode it for the first time since last autumn... it starts reliably now. For the record, I HATE departing from the standard Vespa way, but it was necessary.

This was all about 6 weeks ago. Since my rewiring, I've had zero problems and have put 1200 miles on the bike.

To the OP, I'd say 1. Bypass the relay with the bike turned on. Does the starter motor turn over the engine? Does it crank? 2. When the bike is on and brakes pulled, are you getting 12v on one side of the relay on the control pins? When the start switch is pressed, are you getting ground on the other side of the relay?

My problem was pretty clearly a computer or wiring harness.

p.s. the control portion of the relay will latch with a 9V battery across the control pins.

See above - this is an immobiliser problem (code LED stays ON) not a starter switch problem.