I don't know much about cars so please bare with me.
The check engine light on my Mazda 3 2006 came up while I was on the road. I drove it afterwards for a few days and made almost 500 miles before I took it to an authorized service. Never noticed a single problem or a change in how the car behaved, everything was working fine except that bloody check engine light.
Than in the service guys started playing with my car, they said they were checking a "harness", and than all of a sudden they said that the car is not running well anymore and they will have to replace the "harness" (whatever that is). Fine, they gave me a rental and I drove away.
Took them a week to order and replace the harness, and than today I received a call that the car is still not working fine, that now they think the problem is the computer, and that they recommend replacing that too. Harness replacement will cost me around $600, and now the computer replacement should cost me another $1200.
if there's a light on, there's a code set. they can tell you what the code is and you can see if it's something on the forum.
If they mis-diagnosed many hundreds of dollars of parts you shouldn't be responsible for it.
You may be able to argue that they said it would be fixed for $600 (whatever the first bill was), and its their fault for not figuring it out, and they should pay for whatever the costs to actually repair the problem.
You have to use unclear logic, see if it works. But they could have told you that it is your tires, or anything for that matter, if it didn't have to do with the problem, they should not charge you for it. And they can't prove that it did have anything to do with the problem since it didn't fix it.
Dave '07 M3 GT, manual, - black
Adders: backup sensors, current limited hi-beams (for long life), Fog light on with running lights (t=44332 PAGE2), low beam aiming switch adjustment (t=47722), dash squeak reduction, badges removed, door bumpers removed, auto up/down windows, under seat subwoofer, True hidden hitch (t=53213), rear camber fix (t=53425)
Info / help: outside running lights only as DRL (t=54453)
link example http://www.mazda3club.com/mazda3-mazdaspeed3-98/drl-my-corner-lamp-54453/
i have a feeling your maf or map sensor went bad and your computer learned how to drive with it using an adaptive stratagy, once you clear the codes your ecu assumes everything is good and sees its bad and refuses to start the motor. if that is your problem simply unplug the maf/map sensor and your car should start right up. if it starts then plug it back in and drive away then replace it next week.
What they said, find out what the code was and go from there.
In the future if your CEL comes on just drive over to an auto parts store and ask to have a code read. Most places will do it for you for free, or hold your license and give you their gauge to do it yourself. It takes less than a minute to plug into the OBDII and get the reading back. Once you have the code you can easily look up the issue online.
From the complete lack of information they gave you (and in turn you gave us) it's difficult to diagnose what the issue is. I know a number of people have had issues with the ground on the MAF being damaged and it causing issues, so it's possible that's it. I doubt that and entire harness went bad, but sadly now adays instead of replacing a single piece of faulty wire or a bad connection dealers will spend thousands of dollars on parts and labor to replace entire harnesses. Sad.
When a shop is asked to diagnose & repair, and they quote a price for a repair, the contract is that price will fix it. it varies in different states, in California the shop keeps fixin till its right, max 10% bump for the unexpected after an estimate is given. Did the shop say can't diagnose it till ya change the harness, or you need a harness, as in "that will fix it"?