O2 sensor problem?

A few hundred miles ago my 1999 Dodge Durango 5.9L engine check light came on indicating a bad B1S1 O2 sensor. The original sensor.

I replaced it with a Denso and after about 175 miles the light came on again. This time with a P0135 indicating a problem with the heater circuit or lack of voltage supply. Checked voltage and it showed OK. So I replaced the sensor again.

About another 175 miles and the light came on again with the same problem… P0135. Measured the voltage and it showed OK. Measured the resistance and it showed 6.6 ohms. It seems to be within specs. Out of curiosity, I retrieved the replaced sensor and it also showed 6.6 ohms.

Other than the bad heater circuit, this and the previous sensor work just fine.

I am a little at a loss what next to look at. I don’t think replacing the sensor for the third time does any good. Any ideas and suggestions would be much appreciated.

By the way, the car has about 308K miles on it.

How about the ground side of the heater circuit ?
A bad ground would also give the same code.

I assume the pcm provides the ground for the oxygen sensor heater circuit . . . ?

This one comes with 4 wires,
o2 signal,
o2 signal ground
+ heater wire
- heater wire

With engine on the heater wires on the care side of the connection show 12V, which is correct. I don’t know where the negative wire connects to and shouldn’t make a difference unless there is an intermittent bad connection.

Attempting to find an intermittent connection in this case can be a nightmare. I have not found a diagram that shows where the negative side connects to.

I’m pretty sure the pcm provides the ground for the oxygen sensor heater circuit . . . this is common procedure, if I remember correctly


When testing for the 12 volts, what are you using for a ground to get those 12 volts?
The - heater side of the connector on the car OR some where on the chassis ?

I measure on the car side of the connector. 2 white wires, one negative (ground), one positive. It shouldn’t matter to the sensor which one is what. There is no other ground, besides the signal ground, unless the sensor gets it from the exhaust pipe.

Also measured the positive on the car side and negative (ground) from the chassis. In both cases it shows 12V. That’s why I believe there might be an intermittent connection problem. I plan on “playing” around with the connector some more. That’s really all that was touched when replacing the original sensor.

By the way, I measured the current draw (amps) at the fuse box when engine is running and there is no draw. So, there is a disconnect somewhere.