Diagnosti codes on my 2000 nissan maxima se

what do codes

P0325 knock sensor

p0172 system too rich B1

p0175 system too rich B2

car have over 210k miles, does not idle well at cold start, better with AC on

The rich running engine has likely caused carbon build up in the cylinders. The carbon build up has raised the compression ratio; thus leading to knock which the knock sensor has detected (trouble code P0325).
The rich running engine needs a number of checks and repairs. Among them: spark plugs and wires, air filter, engine coolant temperature sensor check/change, MAF (Mass Air Flow) sensor, and throttle body and idle air control valve cleaning, check/replace O2 sensor, and a couple of other things.

thanks, i did the air filter. what are the couple other things? i was also told i need: valve cover gasket, intake manifold gasket. how much for the MAF? have you heard
of carpartswholesale.com, i checked it for prices and was much cheaper than my local mechanic and they had informative how to links, with estimated savings. the mass air flower sensor is $78 at this site, local mechanic quoted over $560 for the part.

also do you know estimated labor hours for: gaskets, i am told 6-8 hours, most of the costs. thanks again

By, “check/replace”, I mean to do the performance checks on the various parts and circuits which can have a bearing on the running-rich problem. You could, even, apply this method of check/replace to the spark plugs and wires.
This is not a repair you can do by simply replacing suspect parts (at least, not inexpensively). You need a mechanic, or an emissions repair specialty shop, who can, and will, use the correct diagnostic tools and techniques. Unfortunately, this leaves a number of mechanics out.
If you want to pursue the repair yourself, alldata.com has the troubleshooting charts for your car. You can buy a subscription to alldata, or go through your public library (ask a librarian).