02 sensor plug location
 

I have the 2004 1.9T s40 (the model before the 2004.5+ not sure if that makes a difference haha. Took me awhile to figure out there is two different models for 2004 year). Anyway, getting code p0131 and p0420 so going to replace the upstream 02 sensor. Had a couple of questions though. What is plug location for the 02 sensors. What I found when tracing the wires is the farthest right location (passenger side) for the downstream sensor and the one immediately to the left adjacent to it for the upstream location? Was wondering if my sensors were plugged into right spots because I removed the plug for what I thought was upstream and it threw p0036 which is Heater control circuit bank 1 sensor 2. Was expecting Heater condtrol cirtuit sensor 1 code if I unplugged the upstream

And other question is I don't have a swap harness. Saw a swap harness in a Youtube video for another s40 and was wondering if that is for certain years and I didn't need it or if mine is missing.

Thanks for any help!

The one closest to the engine and before the catalytic converter is the upstream sensor, the one after the cat is the downstream sensor. The upstream sensors helps regulate the air/fuel ratio while the downstream measures catalytic efficiency.

Also pay attention to the plugs on the sensors, the upstream might be a wide band O2 sensor (5 wire) while the downstream will have 4 wires.

Thank you for your reply! At the end of the sensor is a plug (not sure what that should actually be called), do you know which position each plug fits on the connector on the firewall? Is downstream plug the farthest right and upsteam next to it to the left? Maybe the plug connectors are unique for each sensor and will only fit into firewall connector one way. My kid has the car so not able to check if that is the case. My other suspicion is that they are plugged in correctly but the sensors are actually swapped in the exhaust system (upstream screwed into downstream location). But then again maybe everything is right.

If your car has the wideband upstream sensor (which it probably does) the plugs will be different and you cannot mix them up. Also, if they are both 4 wire sensors, if they are swapped, it won't make a difference. They are the same sensor, just different wire lengths.

A code does not mean a part is bad. It means the control unit does not like the signal from a part. There are many things other than a bad part that can cause a code to be stored. First thing to do is erase the codes and see if they come back. Or just put parts in that you have not proven are bad - Your choice.

Here are some common causes and mistakes, (from generic sources) and if you unplugged an O2 sensor then got a code for a bad rear heater - I would say you unplugged the rear sensor, not the front.

Common mistakes when diagnosing the P0131 code

• The most common mistake is not confirming that a fault is present in the oxygen sensor circuit by clearing the code and performing a test drive.
  
• Another common mistake is that engine misfires can be caused by a oxygen sensor failure and alternatively engine misfires can cause the vehicle’s oxygen sensors to read improperly.
  
• In addition, a faulty coolant temp sensor can lead to misdiagnoses as well.
  
  

Common Problems That Trigger the P0131 Code

• Defective Oxygen Sensor/Air Fuel Ratio Sensor
• Defective Oxygen Sensor/Air Fuel Ratio Sensor Heater circuit
• Exhaust System Leak
• Intake Air System leak (including vacuum leaks)
• Low Fuel Pressure
• Defective Engine Coolant Temperature Sensor
• Defective sensor wiring and/or circuit problem
• PCM software needs to be updated
• Defective PCM

Excellent, thanks for the input! I'll use that info to help troubleshoot!