I have a 2001 S40 and had recently taken it into the dealership who mentioned I needed a new front 02 sensor. I decided to replace it myself & purchased an OEM Bosch sensor and replaced it. Following the install, I got a number of different codes:
P0171 System Too Lean (Bank 1)
P0130 02 Circuit (Bank 1, Sensor 1)
P0131 02 Circuit Low Voltage (Bank 1, Sensor 1).
The car seems to drive fine, except for the odd time it has trouble starting...not sure if that is a separate error. Randomly, it just clicks when you turn the key and eventually it will start...then days without a problem.

According to Volvo's codes, the P0130/P0131 = Signal is Missing/Signal is Weak.

I've heard that following an 02 sensor install, you have to 'reset the fuel trim' which i'm not sure what this is or how to do it.

Any idea what my problem may be?

 

When you replaced the O2 sensor, did you leave the wiring adapter in place from the old sensor? From what I can tell on my 2001, the wiring is bass-ackwards on the O2 wiring harness. The apparently factory solution was a swap harness that moved the location of the wires in the factory harness to match the wires from the O2 harness. You should see a 12" (or so) harness between the original O2 connector on the firewall and the original O2 sensor connector.

 

When I was first told I had to replace the Front 02 Sensor from my Volvo Dealership, I thought I would check the connectors first before I replaced it. I did remove the 12" extension (from the end of the 02 sensor to the main connector) for a day or two as I thought less connections may be better. That is when the CEL first came on.

When I replaced the sensor, I replaced the 12" extension as it was when I got the car (which was purchased used, so not sure if it was from the factory that way).

Not sure if removing that for a short period messed stuff up. If so, will it naturally reset itself?

 

I would bet the ECM drops the O2 and stays in open loop when the O2 is faulty. It would cause a slightly rich engine and lower your gas mileage. The fastest (and cheapest) way to erase the codes is to drop by an Autozone or Advance (or similar) and have them erase the codes with their OBD-II scanner. They will pull the codes and then erase them for free. They do this hoping you will buy whatever the codes show.

You can trace the color codes on the wires if you are not sure of your connections.

 

I purchased a code scanner a few months back and attempted to erase the codes immediately after I changed everything, but they show up again as soon as I restart the car. I'm notsure if I just need to wait until the drive cycles have been reset or a certain amount of time. I'll try resetting again and see what comes of it.

 

If erasing the codes again does not work, try tracing the wire colors to make sure it's right. Who knows, maybe Bosch matched the original factory wires with their O2 sensor. I bought the generic Bosch O2 and used the original Volvo connector. If I had known beforehand I could easily have wired the generic sensor to match the factory connector and not had to use the swap harness.

 

Does any one have a pic of the o2 with hardness??( maybe a link) and where I may find the o2 senor...I have a general idea, just want verify...

thx.

 

Does any one have a pic of the o2 with hardness??( maybe a link) and where I may find the o2 senor...I have a general idea, just want verify...

thx.