Help with Check engine light codes.

[Deputy1211]

Got the codes read at parts store for my 2001 4runner (2wd V6) as the check engine light and TRAC VSC lights are on.
Test showed P0420 and P0171

Vehicle has 137,500 miles on it. I've recently changed the spark plugs, oil, fuel, air, transmission filter. Is this a problem with the O2 sensor? Haven't changed it since I've had it. Don't know if its ever been changed. Located both of them while under vehicle today changing the oil and am wondering which one (front or rear) would be better to change if that is the most likely avenue I should take towards clearing the light and fixing this problem.

Any info or advice is appreciated and I'd like to hear from a few different folks who've had this issue or experience with solving it.

Thanks

Matt

 

[jsmarine]

OBD-II Trouble Code: P0171 System Too Lean (Bank 1)

P0420 Error Code: Catalyst System Efficiency Below Threshold (Bank 1)

 

[Deputy1211]

OBD-II Trouble Code: P0171 System Too Lean (Bank 1)

P0420 Error Code: Catalyst System Efficiency Below Threshold (Bank 1)

Thanks for the links. But still would like to hear from some of you folks with
3rd gen TR4's that had this same issue and what it ended up being, what you replaced first (or second, and so on), etc.

I see this getting costly if I just start replacing parts without having a battle plan on what order I should try based on experience.

 

[004Gunner]

They could both be related to the exhaust sensors (a/f sensor in front, if cali spec, and O2 sensor in rear). I would clean the MAF sensor first and see if that clears up the P0171. The P0420 is commonly solved by replacing the rear O2 sensor.