Hi, I have a code reader AND a service manual but I still don't know how to fix my car.
I am getting the following codes:
P0140= rear 02 sensor
p0171 fuel too lean

The book lists an exhaustive list of possible causes and I'm not good enough of a mechanic to sort through them all. I had the rear 02 sensor replaced about a year ago and still am getting this code. I also replaced the FRONT o2 sensor about a year ago, just as a precaution but I soldiered it myself and I'm thinking perhaps my crappy soldering job is causing some kind of false reading.

I have no idea about the 'too lean' code. It suggests I have a exhaust leak somewhere but it could also be some other stuff. Any ideas are greatly appreciated. Perhaps some of you have better knowledge on these codes. THANKS!

 

Being that you mentioned soldering, I would guess you installed an aftermerket universal o2 sensor. If this is the case that could be your whole problem. I know that they are sold, but not good to use. When I went through electronics classes for GM, we were told to replace an entire o2 sensor assembly, never solder or repair wiring on them.

 

Agreed about aftermarket O2 sensors. Not a big fan of them myself.
Would try to look for not only exhuast leaks but also vacuum leaks on the intake side.
One common problem that would cause both of these codes to set like that would be a cracked exhaust manifold. Might try taking off the heat shroud and looking for these cracks. 1.6 manifolds notorious for it. A crack near the front O2 sensor in the manifold would dilute the exhaust with oxygen around the sensor and make sensor believe that exhaust is lean untill the point where the P0170 system too lean code would be set. A crack would also affect the rear O2 as that extra oxygen in the exhaust would keep that sensor from cycling into low exhaust oxygen content. P0140 is set and stored if the sensor output indicated high exhaust oxygen content for more that 30.4 seconds when engine rpm is over 1500 and coolant temperature is over 176'F.