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My car gives the code p0131 O2 sensor low volts (bank 1 sensor 1). I changed the O2 sensor thinking that's what it is. It still sets the code though. It only happens when I'm climbing a long hill, then the car loses almost all power and sometimes sets the SES light. Any ideas of what I should look at? Thanks in advance.

Which sensor did you change? Before or after the catalytic converter.

I changed the sensor before the cat.

What brand sensor? Make sure the connector is connected and not corroded, wires are out of the way of the exhaust manifold. Also make sure the plug wires are not laying against it.

Do you have acess to a scan tool that you could monitor while driving?

The sensor brand is Standard. I bought it at A G auto parts. I made sure not to buy a Bosch. I don't have access to a scan tool. It's a strange problem because when it happens and I lose power it is gone until I reach the top of the hill. I can then accelerate with no problem. It almost feels like the cat is intermittently clogged.

I was reading on the gm forum that this code can result from the engine running lean. I'm going to clean the MAF sensor and see if that helps. Does anyone know why a reason why it would only run lean when climbing a steep long hill?

Poor fuel delivery would be my guess.

How old is your fuel filter?

The fuel filter was changed two days ago. I still have the problem though. I thought that changing the filter would fix the problem.

It was a dirty MAF sensor. I cleaned it and haven't had the problem in over a month. Thanks for all your help tracking this problem down.