00 SE 73k

Engine light came on a month ago. Have it checked at Autozone, it was P0171.
So I changed the bank one O2 sensor (an AC Delco one from GMPD).
The light came on again today with the same code. Have it erased at autozone and havn't shown up again after $30 miles till now.

I checked the manual and found some potentials reasons:
1. O2 sensor (which could be excluded)
2. Low fuel pressure
3. Bad connections of PCM, O2 sensor
4. MAF failure
5. Vacuum leak

I am going to replace the fule filter, PCV valve and check the connections. Hope I am going the right direction.

What do you guys think?

It looks like you're going in the right direction. It never hurts to do some preventive maintenance by replacing the fuel filter, pcv valve etc. Install a temporary gage to check pressure on the fuel rail. I'm not sure of what pressure to expect on the rail, I'll try to look it up.

I would guess vacum leak or small exhaust leak.
I got this code while running open header before I got it welded up.

A little bit update:

I replace the PCV valve and cleared the code by disconnecting the battery for a while the day before yesterday. The "SES" light came back on after several trips. Surprisingly, the code went off today automatically . . I did nothing in between.

I will keep an eye on it and report later.

Isn't the same code that some have trouble with and find taking off the gas cap and reinstalling will help.

Sounds like a vac leak (rotting vac line) or a bad MAF sensor.

So far I've replace the PCV and Fuel Filter.
Apparently, they are not the culprits.

The "SES" light went off twice and came on... Weird

Now I am thinking of vacuum leak and MAF sensor.
Doesn't vacuum leak also lead to unstable idle?
I do not have idl problem.

BTW, I think the gas tank should have negative pressure and when you open the gas cap you should hear some whistle as air going into. But I hear nothing when I open mine. Is this related to my problem?

You shouldn't have a negative pressure in the fuel tank when removing the gas cap. If the evap system is functioning as designed the fuel tank should be at "0" pressure, in other words equalized with atmospheric pressure. The only time you would see a differential pressure across the gas cap is when the PCM runs a routine (via the purge & vent valves being aligned) that pulls a slight vacum on the fuel tank. This is with the engine running and certain other prerequisites met, I forget all the details.

Thank you so much Mark.
So, my gas cap "problem" is not a problem.

I will keep going to find out the cause of the P1071 code...

If I follow the sequence here this could be a vacuum failure during the evap test?

Check the gas cap rubber ring that makes the seal with the metal gas tank tube. Coat it with vaseline to give a better seal. Check the part of the tank tube where it makes contact. Rub that with a rough cloth to remove any unevenness to make a better seal. Sometimes a brown powder forms there from gasoline or ?.

Check the tubes going to the evaporative cannister, where that is in the 2000. I had a tube rot at the connection to the cannister. Dealer found it and cut the end off and had enough slack to reconnect without replacing $$$.
On my 98 that was under the air intake tube, under the computer box next to the right front wheel.
The next time I knew the solid light means pollution equipment failure of some type.

The test is run when the car is between 110-160 degrees coolant temp. And is restarted. That was the type of parameters in the service manual, degrees may not be exact. Later I stopped for coffee in the morning and when I restarted the light came on in about 30 seconds. I reseated the gas cap when I got to work. Drove home with light on (cold start, didn't run test). Let car cool. Drove a mile. Stopped for a few minutes. Restarted and the light went off when the test ran.