My 2004 XC70's engine has been fault-free for the 2 years since I bought it. It's the 5-cyl B5254 T2.

After filling tank today, upon restart it threw a P2188

"System too rich at Idle Bank 1".

I noticed right away that the idle was choppy, almost stalling, then racing, then settling down. Driving home, the choppiness continued at
stop lights. It's almost as if the throttle is getting whipsawed.

What does the "too rich" error suggest?