Not sure how much I can stay with this this morning, but.
The po420 is catalyst effiency code. It does not necessarily mean the you need a converter. For example an exhaust leak in front of the converter can cause this code. Typically the CEL would be flashing when this code comes up or just before it.
PO101 has to do with air mismeasurement going into the intake. Look for a vacuum leak between the MAF and the throttle bore first. Clean the MAF with electrical cleaner.
I have no info on PO120 for this truck, but it is a throttle position code on the c series pickups.
Remove the air snorkel to the throttle, Tip in the throttle plate, any sign of carbon and I would suggest doing an intake cleaning and throttle bore service.
My thoughts are that there is a vacuum leak causing 2 codes and a build up of carbon in the throttle bore.
Check the brake booster vacuum line as well. Listen for any hissing to help you locate it and use a can of carb cleaner to spray it short bursts on suspected leaks. The vacuum leak will suck in the carb cleaner and change the idle temporarily.
Friday, November 6th, 2009 AT 7:17 AM