Go with the factory Champion plugs and you'll be safe. At 120K miles, it's time to do it.
You can clear the code by disconnecting the Negative cable on the battery for 30 minutes or so. If it comes back, it's pretty definite you have a faulty component.
One way to avoid seriously gunking up the throttle body in the future is to install a catch can.
Info on the code:
P0406-EGR Position Sensor Circuit High
- When Monitored: With the ignition on. Battery voltage greater than 10 volts.
- Set Condition: EGR position sensor signal is greater than 4.89. One trip Fault.
(K34) EGR POSITION SENSOR SIGNAL CIRCUIT SHORTED TO (F856) 5-VOLT SUPPLY CIRCUIT
(K34) EGR POSITION SENSOR SIGNAL CIRCUIT SHORTED TO BATTERY VOLTAGE
(K34) EGR POSITION SENSOR SIGNAL CIRCUIT OPEN
(K900) SENSOR GROUND CIRCUIT OPEN
EGR SOLENOID ASSEMBLY
most likely cause: EGR assembly