1998 Buick Skylark P0401, P0171, P0420, P0300. All related?

Hello all, I've got a 1998 Buick Skylark here with some issues. Thought perhaps I might pick your brains for some clue as to what I am looking at, and how serious it might be.

I was essentially given this car just recently, so my knowledge of its history is incomplete. I know that it had the catalytic converter replaced several years ago, and the new cat was supposedly a small cheap one. Upon inspection a year ago, the new cat was said to be marginal and would likely need to be replaced soon. It was taken back and forth to the inspection place multiple times because some reading was off slightly each time - either a little too high or a little too low. Eventually it passed but just barely, and I believe that's when the warning about the cat was given. I don't have the exact details about what sensor or statistic was at issue at the time. I'm guessing it was the oxygen sensor behind the cat, but I don't know anything for sure as this is second hand information.

For several months over the past year, the car was used by someone else, who did not take care of it, and in particular did not keep up with the fluids. He put in three quarts of cheap oil right before I got the car, but I don't know if it had had any before that during the time he was driving it. He aparently also left the main air filter box unscrewed, so the car was pulling in unfiltered air. The filter that was in it was dirty, so maybe that's why he ran it that way, I don't know. I don't know how long it was run like that.

Anyway, the car runs, but I haven't driven it much yet as the sticker is expired. I took it to Autozone to have the codes read, to see why the engine light was on. Here is what was on the printouts they gave me:

P0401
EGR system fault. The ECM has detected too little flow on the EGR system.
1. Blocked passage on EGR system
2. Failed EGR valve
3. EGR solenoid faulty
4. Failed EGR pressure sensor where applicable
5. Failed EGR position sensor where applicable


P0171
Fuel trim bank one condition. The powertrain module uses the oxygen sensor to calculate the air/fuel ratio of the engine. The computer has recognized a rich or lean condition on one engine bank only.
1. If bank one and two codes set together suspect fuel pressure or mass air flow sensor.
2. Oxygen sensor defective.
3. Ignition misfire-repair.
4. Fuel injector problem.


P0420
Catalyst efficiency low bank 1. The oxygen sensors monitor the Catalytic Converter's ability to store oxygen.
1. Catalytic Converter defective. Failure possibly due to 2, 3 or 4.
2. Engine misfire or running condition.
3. large vacuum leak.
4. Engine oil leakage into exhause-valve guide seals, piston rings.


P0300
Cylinder misfire detected - random cylinders. The powertrain control module monitors the crankshaft speed and has detected a misfire condition.
1. Ignition system fault - spark plug(s), ignition wires, coil.
2. Vacuum leak
3. Injector fault
4. High or low fuel pressure


These are not in any particular order as they were printed on separate pages, so I don't know in which order they occurred in the computer, or if that is even relevant.

Based on this and a little bit of reading, I have so far cleaned my mass air flow sensor and EGR valve. The MAF has three sensor wires, and one of them was clearly coated in dirt. The EGR valve appeared to be pretty dirty. The opening for the piston (the moving part - don't know proper name for it) had a lot of black carbon junk in it. I cleaned it with a q-tip. Didn't use any carb cleaner or anything like that as I didn't want it to get into the electronics. A quick check of the pins with a multimeter reveals that the pot is working, and the resistance changes when I press the piston down. I don't have a power source to test the solenoid. Obviously this doesn't prove that the EGR is working *right*, but at least it should be doing something.

I also of course bought a new air filter and closed the filter box.

I haven't had time to take it out for a drive since then, so it hasn't been through a drive cycle to retest the codes, and so I haven't had them checked again yet. So I don't know if any of this did any good. I ordered an Elm327 clone scantool off ebay, still waiting for it to show up. If it does before I have a chance to take the car for a drive, I'll read my own codes and see how it's doing, otherwise I'll take it back to Autozone.

So anyway, the thing is, I don't really know what to expect, or where to go next at this point. I know it's probably silly to post this before testing what I've already done, but I wanted to get some ideas anyway. Are the things I've done likely to make any significant difference? Is there anything else anyone would recommend, regardless of the outcome of the minor work I've done so far? I was thinking of basic stuff like changing the oil and plugs, but not sure beyond that, or in what order. I thought of checking / cleaning the oxygen sensors, but I don't think I can get to them.

I also don't know if some sort of cleaning agent like Techron or Seafoam or something else would be prudent. It seems that if the engine was getting unfiltered air and running on too little oil, that things might be dirty inside, but I don't know enough at this point to know what's worth doing.

So, I'd be grateful for anyone's advice or opinions.

First of all If you really want to see this car repaired stay away from Autozone.
Unless you're buying supplies, tires, general repair items but I wouldn't give too much credit to there diagnostics or any of there electrical and underhood parts like sensors. Secondly you have alot of dtcs which may point to more then 1 major failure so before investting too much money in the car to remedy the codes be sure the remainder of the car is in suitable enough condition to warrant any further repairs. You may have a plugged port in the throttle body/intake which can lead to the repeat egr codes. Cleaning the egr doesn't confirm it as okay nor does actuating it. Cleaning the MAF doesn't confirm it as okay either. If the converter is restricted it can cause egr faults, misfires and
the remainder of the dtcs or like I said you can have alot of issues like vaccum leak from the intake gaskets, low fuel pressure, failled ignition coils and modules all causing the converters tofail and the dtcs to remain. This car is a big project for anyone and may be a large bill at a competent shop to remedy. It may be a good candidate for cash for clunkers If you're not very competent in diagnosing and repairring the faults yourself.