1996 Grand AM O2 Sensor doesn't fluctuate

I have a check engine lite on my 96 Grand AM with 2.4 Quad. The computer says its the upstream O2 sensor (code 134) before the convertor. Checking the voltage on it shows a steady .937 to .938 volts. I have another 96 Grand AM with the 2.4 Quad and the O2 sensor on that one fluctuates aprox. .3 to .5 volts all the time. I have changed the O2 sensor and still come up with the check engine lite after the engine warms up and still have the same voltage reading of .937 to .938 volts. Can anyone help me with this?

did you ever change your exhaust or headers?

No, I never changed the exhaust or headers.