O2 sensor low voltage

[Kevmore007]

When I flog the old girl around a hundred miles an hour, foot to the floor, I am getting two codes for O2 sensors under voltage. No codes ever if I drive normal.

 

[Schurkey]

When I flog the old girl around a hundred miles an hour, foot to the floor, I am getting two codes for O2 sensors under voltage. No codes ever if I drive normal.

WHAT VEHICLE???

WHAT CODES???

What is your fuel pressure at 100 mph?

 

[Kevmore007]

1996 GM Yukon PO131 PO151

 

[Schurkey]

And the fuel pressure? A weak fuel pump would lead to excessively-lean fuel mix, which means sustained low-voltage at the O2 sensors.

Are the O2 sensors connected, and have you checked the wire harness for damage?

How old are the O2 sensors?

 

[Kevmore007]

I'm not sure, can I get that from the scanner?

 

[Schurkey]

No, you get that from a fuel pressure testing gauge.

 

[Kevmore007]

I don't know if I have room to reach the port and get the hose in the crack of the hood, then drive down the road at a hundred. I did just change the fuel pump with the finest Chinese delphi one, and it seems to be fine that way.

Also, the tail pipe is pretty black.

 

[454cid]

I don't know if I have room to reach the port and get the hose in the crack of the hood, then drive down the road at a hundred. I did just change the fuel pump with the finest Chinese delphi one, and it seems to be fine that way.

Also, the tail pipe is pretty black.

If the tail pipe is black then you're actually running rich, and the PCM just thinks it's lean. Maybe it only throws a code at 100 because at that point the PCM cant command anymore fuel since the real fuel requirement is higher. If you have a scanner, and in particular can graph with it, look and see what the O2 sensors are doing. They should oscillate between 0 and 1 volt (I think it's 1 volt, Google to be sure).

On my Volvo, the O2 would hang out with low voltage all the time, and the ECM would dump fuel trying to bring the O2 (voltage) up. For some reason the Bosch system though it was a legit O2 reading and kept trying to adjust for a lean condition that wasn't there. I ended up running rich with soot on the back of my car until I replaced the sensor. On my truck the O2 was just dead, and the PCM reported it.

 

[Kevmore007]

And the fuel pressure? A weak fuel pump would lead to excessively-lean fuel mix, which means sustained low-voltage at the O2 sensors.

Are the O2 sensors connected, and have you checked the wire harness for damage?

How old are the O2 sensors?

Sensors are new