Both O2 sensors gone bad?
 

Guys I replaced the front O2 sensor trying to fix a rough-running Protege. Turned out the problem was not the sensors, and now the engine runs OK.

However.. now I'm getting 2 OBD codes, 1170 and 0140. 1170 is for the front O2 sensor (the one I replaced), and the 0140 is for the rear sensor (not replaced). I'm going absolutely nuts with this car. Is it possible that the new one (front) was cheap and has failed and the rear one (original) has also failed.. at the same time? :confused:

The p1170 is more likely MAF or vacuum leak generated. The 2002 model had a PCM flash to cure the p1170. I would ignore the rear O2 code until I fixed the p1170 as the rear o2 code could be related to the front O2 code but not vise versa. Then again the parts we get now adays is generally Chinese junk.

Found this on another place:

Possible Causes for P1170:
Possible causes
- Faulty HO2S-11 Sensor
- HO2S-11 Sensor harness is open or shorted
- HO2S-11 Sensor circuit poor electrical connection
* HO2S (Front) malfunction
* HO2S (Front) heater malfunction
* Fuel injector malfunction
* Pressure regulator malfunction
* Fuel pump malfunction
* Fuel delivery hose clogging or leakage
* Fuel filter clogging
* Fuel return hose clogging or leakage
* Air suction or leakage
* PCV valve malfunction
* Purge solenoid valve malfunction
* Purge solenoid hoses are hooked up incorrectly.
* Ignition coil malfunction
* Insufficient compression
* Engine malfunction


Possible Causes for P0140 (Rear) circuit no activity detected:
Rear O2 sensor deterioration or heater malfunction
Leakage from exhaust system
Open or short to ground circuit between rear O2 terminal A and PCM terminal 35
Insufficient compression
Engine malfunction

Also according to FSM, reasons for front O2 sensor to exhibit low voltage stick (voltage from sensor remains below .45 V):
Sensor malfunction
Fuel injector malfuntion
Pressure regulator malfunction
Fuel pump malfunction
Fuel delivery hose leakage
Air suction or leakage
PCV valve malfunction
Purge solenoid valve malfunction or hoses hooked up incorrectly
Ignition coil malfunction
Insufficient compression
Engine malfunction

Finally, fuel trim problems short or long term, can be caused by inoperative or malfunctioning front O2 sensor as well as by some of the same causes for O2 sensor low voltage stick.

I hope at least some of this will be useful to you. Good luck in finding the problem.

OK thanks. Rock Auto has a closeout and I can get an original equipment Denso sensor for a good price. So, I will install that in place of the cheap one I put in, take a good look for vacuum leaks, and see how it goes.