Head light replace ,,now the car sometimes will start sometimes it won't help !

Question edited for a lot more clarity. Is your space bar and Caps Shift broken?

https://www.google.com/search?q=2007+Cadillac+STS+3.6+6+cylinder+purge+valve+clip

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Here's a google link so you can see what they look like and prices.
https://www.google.com/search?ei=yj5CXPTWDM2Bk-4P_pm5gAo&q=2007+cadillac+sts+door+lock+actuator&oq=2007+cadillac+sts+door+actuator&gs_l=psy-ab.1.0.0i22i30l2.282.4490..6438...0.0..0.541.3634.0j4j8j1j0j1......0....1..gws-wiz.......0i67j0.QNGBcug4E1o

go to start black screen ??? Need more info from you . What screen goes black . What light's up ? The instrument panel ? Do the headlamps light ? Do they go dim or out when trying to start the vehicle ? Is the battery good an have good cable connections .

You have a severe engine misfire , take it to a repair shop ,have it towed ! Don't drive it any more , will damage catalytic converter ! Just had to replace one on a vehicle because owner drove it , cost $800.00 .

Probably a crankshaft position sensor problem. Is the check engine light on, and if not, does it EVER come on? People sometimes do remove trouble lights to sell troubled vehicles!

Sound like the ANTI-THEFT system or PASS-LOCK system is armed; in your Cadillac Owners Manual (need yahoo register for free) could find this and other details...

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Hope helps (remember to rate and comment this answer).

most of the time it occurs after replacing a battery but if this occurred out of the blue then something like the transponder has failed

you'll need a professional scan tool and about an hour to diagnose the concern

then when fixed, you'll need another 30 minutes to reprogram the computers and keys

easily mabe. look down from under the hood, if not try try from underneath.

I had the same problem with my STS. Go back through your programing menu and "check" remote start again. Somehow, mine defaulted to not allowing remote start feature. It was good to go as soon as I re-programmed it.

you need a scan tool. go to auto zone they will do it @ no charge good luck.