Hi,
First time posting and new to Mazda.
I scanned the CEL and found P0031. That number is not listed on the code book. I found in this forum, it is O2 sensor bank 1, sensor 1. Which one is it? Before cat or after cat? The one by radiator or by firewall? How many wires is the O2 sensor. I found in Ebay with 4 wire front or back for $25.
Thanks

 

I've got a 2000 MPV that's showing the same code. This indicates the upstream sensor near the radiator. Unfortunately, I changed my sensor and I still get the same code. I'm waiting for a reply on another posting.

 

The P0031 code is indicating a fault in the bank 1, sensor 1 O2 heater circuit. The heater is just used to warm up the Oxygen sensor as quickly as possible as the operating temp is 600-700'F. Typically when this kind of code is being stored it is time to replace the sensor as the internal oxygen sensor heater has more than likely burned out. This sensor is located in the exhaust manifold on the right bank of cylinders of the engine. It is sensor before the warm-up cat. Not the side near the radiator but the side closer to the firewall or windshield. It is a 4 wire sensor. Perhaps this is why your code came right back JoseC as it sounds like you unknowingly replaced the wrong side. The sensor located in the manifold closest to the radiator is Bank 2, Sensor 1 and would have stored a different code altogether. The sensor is a 4 wire, 2 for the heater circuit and 2 for the Oxygen sensing portion. Would be a little wary of a $25 sensor as this is the primary control input for the fuel amount control of the right bank of cylinders. Going cheap may seem like it is great in the short term but if the sensor is not a quality part it could end up costing you way more than that in fuel costs. At almost $3.50 a gallon I would want to utilize it as much as possible.