PO171

O2 is not the issue. 34% long term is the issue.

You have a vacuum leak somewhere.

The causes could be vacuum leak, pcv valve, mass air flow sensor or low fuel pressure. One of these will the cause. After you repair it, clear the memory so it starts the long term count over again.

Roy

My 4runner 2000 used to have p171 code at 90k in 2008 ago and I found the fix online but my technician do not listen to me in the beginning. After he failed to fix it for two times, instead he replaced O2 sensor according to the repair manual. Every time he reset the memory, the check engine will come back after 50-100 miles. The fix on the third time was simply by removing mass air filter MAF sensor and we found a thin hair short cross the two metal wire inside of it. Clean it by a blowing airgun and put it back. Now it has 137k on it without further problem. Be careful when removing MAF filter because two screws are easy to be damaged and we actually drill one off- this was also pin pointed by that original post maybe from yoto-tech BBS.

I just solve the problem, . Problem was the vacum line from fuel pressure Reg, I pull out the vacuum line from the fuel pressure regulator and find out that the line was sucking so much air, so I check on vacuum diagram and found it was the line was route it wrong