Scotty, I have a 99 Silverado with 345000 miles on it. My check engine light came on just last night, so I ran the codes and I got PO157, O2 senor voltage low. I watched the live data on my truck for a couple of minutes, and the only abnormal thing was that the LT fuel trims were between 14-15% and ST trims were at about -2%, but the voltage was running about .04V, all of the other sensors were adjusting fine. (I do have a winter time only air leak, only affects the engine if it goes below 0 for too long).
I replaced this specific O2 sensor over the summer, probably around June. At that time, I had the code PO161 floating around for a few days. Replaced the sensor with an OEM sensor and it has ran fine until last night. Could this be a sign of a bigger issue? I would not think it is, especially because everything is made like crap nowadays, but I feel like going through 2 O2 sensors in 7 months is a little unusual, and its the same sensor, Bank 2 Sensor 2.
Let me know what you think. Thanks!
Your scan data is showing that rear O2 sensor at 40mV and that will set the low voltage code.
And your fuel trims are high so the upstream O2 sensor is seeing too much O2 in the combustion gases.
Or is it?
I'm not a fan of addressing O2 sensor codes without correcting the reason for high fuel trims but here we go. Maybe you can kill 2 birds with one stone without going down a rabbit hole.
If those high LTFT (trims) are the result of an intake vacuum leak somewhere then the cause of the high O2 levels being seen by the upstream O2 sensor (high Positive fuel trim) is indeed in the combustion gasses.
But what if it's an exhaust leak? The computer wouldn't know that and it will add fuel in an attempt to correct the Lean air/fuel mixture.
Of course adding fuel won't fix the exhaust leak.
On the other hand, it could be a wiring issue like @chucktobias mentioned.
Maybe rule that out 1st since you have a scanner and you won't even have to crawl under the truck for this one. Here's the setup on yours. The Purple wire is the HO2SB2S2 high signal wire and the Tan wire is the HO2SB2S2 low signal (computer ground) wire.
The way that rear O2 sensor voltage would be getting pulled down that low is if there were a short to Ground on that Purple wire or if the O2 sensor is shorting to ground internally. (not taking into consideration any possible issues with the PCM itself)
Hook up your scanner, Live Data, turn the key to ON, and look at the voltage value from that B2S2 O2 sensor. It should be very close to 450 mV. If it is, there's No short to ground happening here. If it's being pulled down to low voltage at key on/engine off then there probably is a short to ground.
If the voltage is good then you want to force a Rich condition by spraying some starting fluid into the intake while watching that B2S2 voltage to make sure the sensor is working. So if it's at 40mV (engine running, fuel system in Closed Loop) and when you spray starting fluid into the intake the voltage rises by a couple hundred millivolts then you know the sensor is working.
Could it be a vacuum leak causing this? Maybe, but not so fast.
You want to see if there's an exhaust leak. Your exhaust system is setup like this
I highlighted the exhaust donut flange in orange. You want to repeat that same test you did with the starting fluid, spraying it on the exhaust donut flange while someone is watching the B2S2 voltage to see if it rises. (you're just replacing any O2 being sucked in through there with the starting fluid). If the voltage rises then there's where the problem is because if that seal is good it won't be able to suck any starting fluid in through that donut flange. You want to do the same thing where the exhaust manifold mates to the cylinder head.
(you're using flammable stuff so keep the garden hose close by just in case)
In cold weather metal parts don't expand and seal gasket leaks as quickly (if at all) as they do in warmer weather so these issues tend to be more problematic in cold weather.
I would carefully check wiring, connectors, and grounds. They could have deteriorated over the last 24 years.
it has ran fine until last night
what happened last night?
