Yikes, 5% CO. The CO2 is dismal too. Rich is an understatement but there's something weird going on because the O2 indicates a lean mixture. I'm guessing your exhaust system has seen better days. It's for sure rich though, the high CO and low NOx prove it.
Congrats, you're deep into the Gross Polluter category. Your car is the reason smog laws exist. It also shows why the system as setup sucks because you've likely been driving it that way for a long time. If I may rant for a moment this is why MKIIIs are Test Only in California. Everyone has to deal with the Test Only hassle when too many fail because people who don't know what they're doing screw with or fail to maintain them properly. List your mods.
You're obviously not running in closed loop because if you were even with a dead cat emissions would be better. That said depending on how long you've been driving it you may need a new one even after the root cause is fixed. It's amazing you don't have any codes other than 51 but it isn't the cause of your richness. You should fix it though. Check the TRSM or search here for info.
Could be lots of things. Fuel pressure too high, timing wrong, dirty air filer, bad AFM, coolant sensor, O2 sensor, exhaust leak near the O2 sensor, on and on. Put a meter on Vf and check both the long term fuel trim and O2 sensor activity then go from there. If that's Greek to you take it to an emissions tech. The problem is so bad it shouldn't be very hard to find.