Hey guys i really need help with this 1995 buick lesabre i have replaced the ignition switch with a new key with the chip that matched the old one. It is still coming up th P0629 pass key frequency invalid i dont know what else to check it starts like for one second and then it turns off. I was thinking the decoder module but dont know i really need help. thanks

You could do a VATS bypass.

https://www.gmforum.com/electrical-1...2-99-a-279119/

i allready tried that and it didnt work but now with the new switch and key theres no need for that

Perhaps the VATS module has failed. This is the device that read the resistance on the chip on the key.

Under some conditions the vats module has to relearn the key.. This is done by turning the key to the on position and wait until the security/theft light goes out.
Do this three times and it relearns the module.. After that the car MAY start.

ive tried relearning it too and it didnt work i really think its the decoder module but i cant find a way to test it if its that

Someone on here used a Multimeter that had the capability to insert a 50hz signal via the probe on the PCM side of the car and was able to start the vehicle. But I don't remember who or what though.