O2 sensor/check engine light help please
 

Hey guys. Any help appreciated. i have an emissions test I have to pass by april 30th. And i have the check engine light on. in the past i have had mechanics clear the codes and that has worked . No longer. my codes at this point are
p0170,p0173,p 0156,p0125,p041.
so far I have replaced all 4 o2 sensors. Did not help. I bought the extenders to try to cheat the reader. No dice. One post cat sensor is no longer reading voltage. Yet there Is voltage coming from source so I have ordered another replacement sensor.
im getting frustrated and worried i cant pass.
i know o125 is a possible coolant sensor issue. Is that easy to locate? I can replace that to try to remedy issue.
i know o141 also involves the o2 sensors. Do I need to again replace the whole damn set? I bought walker product sensors. Was that a mistake? Can the sensor without voltage also trigger pre cat fuel trim codes?
my mechanic who works out of his home on the weekend has not been able to figure it out yet. Am I doomed to need to go to a shop and spend a fortune on their looking for the problems with way better equipment?
i love my monty (2001 limited) and with all the money I have sunk in, I dont want to junk her. But i have my limits. Any help or suggestions massively appreciated.

You've got much more going on there than just sn O2 sensor(s). Two points :
1. Simply clearing the codes does not fix anything.
2. Stop blindly tbrowing parts at it.
If your mechanic can't figure it out beyond just clearing the codes, take it elsewhere.

The truth of the matter is, if you or your mechanic can not figure out the problem, you need to take it to a professional who will fix this. Yes is more expensive, but I think is the best solution. Just make sure you take it to a mechanic that know Mitsubishi motors. If you are located in socal I recommend Ozzy.

My mechanic is a professional. He works in a shop 5 days a week and does work out of his home on the weekend. Of course,he has little of the testing equipment a shop has. I cant afford a shop at this point so we will keep trying to find the issue(s). Thx anyway