I have a 98 Mistake too and have had many problems. I have replaced the IAC valve, repaired a vacuum leak on the brake hose, two bank 2 O2 sensors, and now had problems where it would not start all the time. Found out that the fuel pressure regulator was bad. The codes it shows are not always easy to read, they can mean multiple problems are possible but only one of them is causing the fault. I'm a pretty good mechanic but sometimes the problem is hard to track down. Buy a good manual and go through all the steps to diagnose it, it is a pain sometimes but if you follow the manual and have the tools it can be repaired without just swapping parts.
The p0171 code is what mine was showing, it says lean condition but what was happening if that the pressure regulator was bad allowing too much fuel pressure that would flood the engine so the PCM would switch it to lean mode to try and compensate. Some cylinders were flooding while others were lean so the engine would not start. I tested the fuel pressure to find this.