My car has just not started, no clicking when I turn the key, no noise, nothing, but my dashboard lights go on. It did this back in January and it was in the garage for three days and would start right up so my mechanic said he couldn’t diagnose it unless it wouldn’t start. Now last week it happened again months later and so I had it towed there so it would be dead and when they took it off it started right up! I left it there for five days and it was starting up at least a dozen times a day. This is frustrating to say the least. I just never know when it won’t start.

Any suggestions or thoughts on what may be causing this to happen?

Thank you in advance.

Turn the key and no joy.
I had this happen with my old 740, a frustrating intermittent gremlin.
Then it would start up immediately with a screwdriver across the solenoid lugs for a second and it would work for minutes or months.

I tried everything including swapping the starter, but the problem remained.
It's obvious that the solenoid is not getting enough current to start via the normal key circuit.
Why it usually resets after shorting the solenoid lugs is the $50 question.
The solenoid may hang up requiring more current than the key circuit offers?
The interlock switches and ignition switch may limit current, intermittently. How to tell?

here's a good idea from: https://forums.pelicanparts.com/porsche-911-technical-forum/196077-how-do-solenoids-fail.html
To make sure it was not an ignition switch problem, I connected a test lead to the proper terminal at the starter, and placed a test light on the passenger seat. That way, I could watch the test light as I operated the ignition switch. The test light turned on, and the starter did not operate. Bingo, bad solenoid or starter. And again, the starter itself was fine. Frankly, starters just operate a few seconds at a time, then they get to rest. Alternators are continuous duty. So, starter motors can last a while. Solenoids are the problem at least 80% of the time that starters don't work. And as I say, a new Bosch solenoid is probably just around $30.

sounds like it would make sense to do the test light thing, to ensure that the problem is the solenoid rather than the ignition switch. (and interlock switches)

Finally the 850 has a service socket in the engine bay,
It is simply a wire to the starter solenoid with a molex connector. Find out if the S80 has this service socket. If not your mechanic could install a heavy wire to the starter solenoid. Then when the key fails, you leave the key in the run position and put +12V to the service socket to start the car.
If you can see the solenoid terminals just short them with a big screwdriver
to do the same thing. This is preferable to the time and expense of waiting for a tow.

Finally there is also the possibility of the alarm module causing this problem since it has a set of series contacts in the starter circuit.

To simplify:
The test light for the solenoid could be run from the service socket,
which be a good way to find out what's wrong. It doesn't start and the light comes on, you have a sticking solenoid (very possible)
If it doesn't start and no light comes on, it's the series switches or ignition switch.

Good Luck, Bill

Thank you Bill. It starts every time my mechanic puts the starter tester on.
Would that be the same as the part I need to tap with a screwdriver? If not where is that part located? Pardon my ignorance but I’m not a mechanic am just trying to get this demon out of my car so I don’t have an anxiety attack every time I need to start the car.

I don't have an S80 so I don't know if you can see the solenoid lugs.
How to make a service socket if you don't have one didn't I already explain?
install a heavy wire =<14 gauge to the solenoid lug terminating in a female Molex pin (get one with an insulator since you cannot let this wire randomly touch ground anywhere. Secure it safely somewhere to keep it safe with a ty-wrap or tape. Then make a jumper to reach the battery, I suggest that you get a suitable alligator clamp to grab the + terminal of the battery and a small alligator clamp on the other end that can grab the shank of a small screwdriver that can be poked into the New Service socket to start the car.

Have the service socket several feet away from the battery, as Volvo does in the 850 since you will get some sparks when you do this, and you don't want sparks near a lead acid battery since there's a chance of explosion.

If you don't understand my posts, just print them and show to your mechanic.

Bill

I do not know about newer Volvos, but 850s suffered with dirty PNP(Park Neutral Position) switches.

These switches are quite complicated and they determine if you can activate the starter, in Park or Neutral.

Usually their troubles come on in cold weather, one way to temporally clean the switch is to hold the button on your shift lever and row the shift lever back and forth over its full range ten times or so.

If this works, your PNP switch is on its way out.

Do a Google search for - Volvo PNP switch - select images - go down a page or two to see the inside of an 850 switch - quite a mess!

A lot of things can happen on a 17 year old car. It could be the ignition switch, a bad wire connection from the battery to the starter, the starter ground wire, a computer that won't wake up, a security module with a dead battery, or even a weak battery in the engine bay.

Do you park indoors? How many miles on the 80? Do you use synthetic? Did you try to shift to "N" and try to start the engine?
--
Keeping it running is better than buying new

Wow...that’s a lot of things that it can be!
It has 128,000 miles on it, I do use synthetic oil, and I have tried jiggling the key and putting it in neutral. It’s just so strange how it happened months ago and then was starting fine and then just wouldn’t start again.

I’ll pass along what you mentioned to my mechanic and see if he can check them out.

Thank you for your input!

Obviously, your problem could be numerous things.

Besides turning the key, have you tried other things?

Will the horn blow when the car won't start?

Do the directionals work?

The headlights?

Do you have a heavy key chain? Have you tried wiggling the key?

When the car won't start, tap the battery connections with a screwdriver handle as if you were trying to turn them around the posts. This may change their connections.

Yes, everything works when it doesn’t start which is what puzzled me.

I will try tapping the battery connections with a screwdriver next time it happens and hope that helps.

My keychain was heavy but I lightened it up after the first time it did this.

Thank you for your suggestions.

It happened again today.
My mechanic came to where I was and when he put the tester on for the starter it started right up!
I passed along all of the info that I’ve received and he’s going to have me leave it again but he’s pretty frustrated, as am I. More to be revealed. 🙄