CAR REPAIR

This is the WORSE SERVICE DEPARTMENT IN EL PASO! I have had to bring my husbands car back to them since last November due a check engine light that would not go away. I brought it to them, supposedly it was the catalytic converter, got it replaced, please drive it for 100 miles and the light will go off they said. It didn't of course. The car drove great, except that light wouldn't go off. So I drove it for 113 miles, light is still on. Called back, they said drive another 300 miles. Light stayed on. I take it back, they told me "OH no, drive it for 1000 miles. Are you kidding me? So again, 1000 miles and still light is on. Took it back, they gave me a rental to drive while they figure out the problem, for another MONTH. They kept saying they didn't know what as wrong with it, car runs great! They will call GMC and see what they say. I get a call, They said " Oh it's your fuel injector, running rich. We need to replace them so your other catalytic converter doesn't get messed up." Took them ages of course. Charged me $600+ and I only agreed coz we needed it done. He's an Infantryman, in the Army. He's busy as hell. Fix my car coz he doesn't have time. We finally get my car and BAM two days later it starts acting up, shaking, hearing knocking noise. I take it back, angry, asking them what the hell they did to my car. One advisor said, "Hmm been in her awhile, with all the diagnostics it's bound to have communication issues." I don't hear from them til 3 weeks later. We don't know if its the engine blah blah blah. I had to had to call again. I get my answer another week after, Hubby was in the field, and they tell me $7600 for a new used engine on my car that barely has mileage, just paid off, had them work on it since Nov. BUT I GOTTA PAY $7600 for a new used engine??? My car was running great until they started messing with it!