I have a 2003 Protege 5. The "Check Engine" light is on. Reader pulls code P0031, bank 1 sensor 1 O2 sensor problem. When looking at the motor there is a sensor upper right, then there is another one lower left under the car. Which one of these do I need to replace?

Thanks,
Ryan

 

P0031 - Oxygen (A/F) Sensor Heater Control Circuit Low (Bank 1 Sensor 1).
A P0031 DTC (diagnostic trouble code) refers to the O2 sensor (oxygen sensor) located on Bank 1 in front of the catalytic converter (first one, just before converter).
This O2 sensor #1 may also be referred to as an air/fuel ratio sensor since on some vehicles it is. It detects the amount of oxygen in the exhaust gas compared to the outside air and then vehicle's computer adjusts the air/fuel ratio going into the engine. The sensor is less effective when the exhaust gas temperature is low, so it includes a heater which is activated to help get better readings from the A/F O2 sensor. Essentially this P0031 code means that the resistance of the heater circuit is lower than normal. In most cases, that resistance level must fall below 0.8 A to trigger the DTC code.
If you follow exhaust flow from engine, it's the one before the converter. After it goes thru converter there is another one, that is sensor 2.

 

Ok thanks.

I did replace that sensor (the one right after the bend in exhaust) and cleared the code. After the second start up of the car the light comes back on and same code.

Intially I had a second code P0117, the ECT sensor, replaced that sensor and that code has not returned.