This car has had problems ever since I bought it. I have had an annoying hesitation when I take off from stop signs and lights, it usually drives fine when I am at speed, has decent acceleration for an old volvo. I am getting a lean/rich code, 2-3-1, so I thought that maybe it was the o2 sensor, I actually unplugged the sensor yesterday to see if it drove any better and it did, no hesitation anymore, so I thought for sure that this is the problem. I was wrong, the car is still hesitating, although not as bad now, only for a second or two. I wish I could just unplug the sensor but my mechanic said that this could ruin the catalytic converter. I have replaced the fuel relay, the MAF sensor, the fuel pressure regulator, the idle control valve, o2 sensor, checked the air filter, the fuel pump seems to be working fine, the car is getting spark, I have also unplugged the fuel injectors while the car is running to see if the idle changes, which it did. What should be the next step? I would like to finally solve this problem. Thanks, Jay

You'll want to measure the voltage on the o2 sensor green wire - it will tell you rich or lean.

My guess is you have a vacuum or exhaust gasket leak, leaky injector o-rings, a hole in the hose between the AMM and throttle body, or a split vacuum line.
While the car is running spray these areas with WD40 and see if the idle changes, if so you found the leak.

Some personal experience:
I used to own a 90 non-volvo with a cross threaded PCV valve directly into the aluminum throttle body. The idle would jump when I sprayed it with WD40.

This weekend I worked on a 95 non-volvo with a check engine light and O2 sensor trouble code. I found a split vacuum line off the intake (that controlled the EGR vavle). I fixed line and the code went away without being reset! The car also had hesitation but would idle OK ...

Have you cleaned the throttle body? Maybe a plugged catalytic converter???

Problem seems to be gone now, maybe it just took some time for the new o2 sensor to start working. Hope its fixed. Thanks.