Couple of engine codes for your reading pleasure...

Hi all, my '94 4x4 has been acting funny and lighting up the CEL so I took it to Advance Auto Parts and got the codes read.
Here's what I got:

KOEO Code 335
EGR PFE/DPFE Volts
Out Of Range

KOEO Code 539
A/C Or Defrost On <--- I know I can easily fix this one by turning
During Self-Test them off.

Memory Code 172
HO2S (sensor)
System Lean
(Bank 1)

Memory Code 176
HO2S (sensor)
System Lean
(Bank 2)

Memory Code 181
No Definition
Available

Memory Code 186
No Definition
Available

Memory Code 189
No Definition
Available

Memory Code 211
Profile Ignition
Sensor
Circuit Malfunction

Memory Code 556
Fuel Pump
Primary Circuit
Circuit Malfunction

Now, talking to the guy at AAP he said to start with the egr valve since that might trigger all the other items on the list. What do you guys think? Is there a site that has pics and details about where the egr valve is and how to check it and clean it? Thanks in advance for any help you can give.

Brett