Oxygen sensor problems I would appreciate some help with

I have a 2005 Impala with the 3.4 engine and 118,000 miles. About 2 years ago I had a code P0141 and since we didn’t know for sure which oxygen sensor was out we replaced both of them. The P0141 code did not stop occuring after both sensors were changed. I read on an Impala forum that if you didn’t replace the sensors with AC Delco the codes would continue so I didn’t worry about it until my gas mileage starting going down recently. I ordered new AC Delco oxygen sensors and my husband replaced them Saturday, the old ones were sooted up pretty badly. Today a new code appeared, it is P0140, O2 sensor circuit no activity. I would appreciate any ideas on what the problem might be now.

None of those codes can tell you that sensors are bad. They all just refer to data the computer has - and many of them are just about circuits. The sensors are part of the circuits - but then so is the rest of the circuit. The A/C Delco thing is probably bunk and you probably replaced perfectly good O2 sensors.

Here are some fairly clear descriptions of the codes you report. Someone needs to check the wiring for the sensors.

To tell the sensors apart, Bank 1 is which ever side of the engine has cylinder one on it. This can be figured from a diagram of the spark plug firing order. Your car has 2 banks, so Bank 2 is just the other side. Sensor 1 is the one before the catalytic converter (a.k.a. upstream). Sensor 2 is after the catalytic converter (a.k.a. downstream).

But from now on don’t just assume that you replace a sensor just because a code refers to a problem in the sensor’s circuit.

Also, this car is a V6. As Cigroller implied, but didn’t elaborate on, this car has 4 oxygen sensors minimum, two on each side of the engine, one for each bank before the catalytic converter and one after each converter. Some cars have 5. The two codes are pointing to the same sensor, the rear sensor on bank 1. Bank 1 is the same bank that cylinder #1 is on. The rear sensor is behind the catalytic converter. You probably replaced the two sensors Before the cats, because you can see those from under the hood. The rear sensors are underneath the car further down the exhaust pipe. Those are the one’s you probably missed.

Ah yes - I’m sure that BustedKnuckles has guessed correctly about that. I read right past the “replaced both” - as in two. In which case, I’d also bet that you replaced the upstream sensors when the codes have nothing to do with those circuits/sensors.

My car only has one catalytic converter. There is only one oxygen sensor after that converter. Is it possible that there are 3 oxygen sensors? 2 upstream and 1 downstream? I have received both P0140 & P0141 codes since I first posted. Since both of those reference the downstream o2 sensor and we replaced it, am I safe to guess that it’s a bad sensor out of the box or is there something else to check?

It’s a bad sensor, the wrong sensor or damaged wiring to the sensor…

ImpalaELF — "My car only has one catalytic converter. There is only one oxygen sensor after that converter. Is it possible that there are 3 oxygen sensors? 2 upstream and 1 downstream?"
The GM 3.4L engine has only two O2 sensors, one upstream and one downstream from the cat. The 3.4L has no Bank 2 sensors, only Bank 1 Sensor 1 (pre-cat) and Bank 1 Sensor 2 (post cat). Your problem is with O2 sensor Bank 1 Sensor 2 (post cat).

Here is some info on DTC P0140: P0140 - 02 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)

Check for

  • 2-3 Ω across sensor heater;
  • ≈1 V sensor output with sensor tip in propane flame;
  • short-to-ground in harness wire between sensor output and CPU; and
  • continuity in O2 sensor heater fuse.

In the past whenever I had replaced an O2 sensor with a new one… and then received the exact same code for the same sensor… I always had a bad wire from the ECU to the sensor…Wires in our vehicles with their complex routing and even more complex plugs attached at the end…for some crazy reason GO BAD… I mean on the vehicle I have in mind (a Nissan Maxima) the car was in every way perfect…no past trouble…no accidents…nothing. When I replaced the O2 sensor that the code pointed to (2X) and then got the same code (many X)…I found a faulty wire from the O2 to the ECU…there was no external damage to the wire and nothing that would make you suspect that wire/connection (except this issue)…BUT SURE ENOUGH…the wire was bad/broken somewhere for some reason…

 I ran a new wire from the O2 to the ECU and BANG.....Everything was A OK....  SO.....  TEST those connections.....grab a pinout of the ECU and figure out how to do a continuity reading between the start and end.... (you can focus only on the O2 wiring...shouldn't be that many wires to deal with and the test goes fast....)  Check your wires if you cannot get the results you expect with part replacement.....  

Just a little background story…All true.