O2 sensor diagnosis

[goldenreed]

My first post to the site but frequent user...
My 99 V70 (non-turbo) 185K miles had the usual CEL and codes P1030 and P1035. I have access to a simple code scanner/reset but not a full service scanner. When I reset the codes come back on within a minute or so. Over the past week, I've since replaced the front sensor twice (one cheap aftermarket and then a Bosch Universal since they were both available locally and had heard warnings regarding the cheap ones on volvos). I've tested both units off the car and they seem fine - good current through the heater and appropriate voltage swings by multimeter in response to fuel.
With the Bosch I've back-probed all 4 wires while on the car. The results surprised me and I hoping you can tell me whether this is normal ECU function. With codes cleared, on startup the voltage goes immediately to .38V and is stable there (heater 12-13V) and usually starts fluctuating as expected. Invariably with 1-3 minutes or so the sensor voltage drops to .000 mV and heater voltage (vs ground) goes to 0V. Based on repeated scans during this time looks like a P0130 or P0135 code comes up before the voltage goes south.
Code reset while the car is running will return the voltages to normal for another 1-3 minute cycle. I would have thought that the backprobed sensor would still send a fluctuating .3-4V signal after code.

Is there a relay to control the circuits that may be failing OR does the ECU short the sensor values leads (and open the heater) and hold it until reset. Is this fault likely in the sensor (poor following)? or a problem in the ECU or wiring upstream?

If this is normal cheap sensor response, I'm ready to buy the OE (Denso) direct plug O2 sensor from amazon ($140) but wince at the $330 my dealer quotes for the "Volvo" one. If the symptoms suggest a central problem it may be going to the shop unless there is a clear fix. I'll wait to spring for a new sensor based on your advice. Any reason to replace the rear sensor if no codes?

[jblackburn]

Usually the most common cause of the O2 sensor codes is actually a vacuum leak.

Since you have replaced the sensor...twice...and are still getting the code, the sensor is definitely not your issue.

Start looking for cracked vacuum lines. There is a common one that cracks away behind the alternator & PS pump that can cause this code, as well as the ones at the vacuum tree yourself.

There would be no sense in replacing the rear one if there aren't any codes for it, since it doesn't do anything but monitor the efficiency of the cat. converter.

[jking4020]

What about the tranny bad soleniod part? My 02 xc70 has issues accelerating in higher gears. Slips a bit. Its still fast as hell but in higher gears the tranny slips. So i have to be gentle. Anyone have info for that? P1035. Ill look for vac leaks. Check #1 coil for fault. But the tranny is bad. Oh how i dont want to loose this car. Its SWEET!

[Clemens]

Hi!
You´re in the wrong forum. This is the forum for P80 cars up to 2000. You will most likely get more feedback in the right forum.