I am working on:

2001
Nissan Sentra
1.8 L

The "Service Engine Soon" light comes on repeatedly
soon after being reset. The DTC is:

P0154
Oxygen sensor circuit no response detected (Bank 2 sensor 1)

The oxygen sensor is, in fact, capable of switching.
It does this rarely, because the fuel control system
remains in open loop on bank 2. A new oxygen sensor
did not fix the problem. The new sensor switched
between low and high as the old one, and about as
infrequently.

The weird thing is that the scanner says that the
output from the oxygen sensor is a constant 1.275
volts. Or on some occassions it stays at a constant
2.65 volts. The oxygen sensor is not capable of
producing so high a voltage. And the Vantage trace
shows the real output to be in normal range.

What could be the cause of such a problem? Is 1.275
volts a substituted value? If so, under what
conditions does the PCM do substitution?

A couple of other tests. (1) I can disconect the
O2S wires and test the voltages on the PCM side of
the connector. None of the wires are shorted to
1.275 volts, as seen at the connector.

(2) I can unplug the harness going into the PCM. I
find continuity in the signal wire from the O2S
connector to the PCM connector. But there is one
more weird thing here. The pin number at the PCM is
one-off from the pin number specified by SomeData :-).

So what could cause a good O2S to give a faulty
reading at the PCM, as seen from the scanner?

Jim S.

a short, or the pcm itself. theres nothing else in the circuit. make sure your DLC connector is operating properly also...