P0135 - P0155 Volvo S80T6

Volvo 1999 S80T6 Check engine light came on. OBD II codes P0135 along with P0155. Bad wire?

Both codes are for heater circuits in the Bank 1, sensor 1 and Bank 2, sensor 1 O2 sensors. Has any work been done recently? It would be rare for both of these Heater circuits to malfunction at the same time. Has your MIL been illuminated for a while? Or only recently?
Check both front O2 sensor harnesses visually. If you don't see anything abnormal, and no work has been done to the vehicle(and you haven't hit anything) then post back and I or someone will walk you through how to check the O2s.

No work has been performed on the car since last summer. Just regular oil changes, 3K. I felt the same as you, a little strange both would send a code. That's why I was wondering if a fuse had opened. Checking this circuit is next to impossible. I would just change them if I'm going down that road but at 160.00 each I thought I would pose the question.

Yes, it is very likely that a blown fuse could set both codes. But the real question is: Why did the fuse blow? There are a couple possibilities. 1. A malfuntioning heater circuit or 2. A short in the wiring to the o2 heater. Note that a short in just one of the O2's heater circuits would set a code for both because both are likely fed off of the same fuse. So check fuses. If an O2 fuse is blown=check your wiring visually for shorts. If you find none, then unplug both front sensors and then install a new fuse. Turn the ignition on. If the fuse doesn't blow immediately then the problem is in one of the sensors. Now plug in the sensors, one at a time, checking the fuse in between. When the fuse blows, the last thing you plugged in is the culprit. Hope this helps.

OK, Checked the fuse block in the engine compartment. YUP its a fuse just designated for the O2 sensors. Replaced fuse cleared code, didn't come back on. Looks good, Ill keep you posted. THERE IS NO WAY TO PLUG & UNPLUG O2 SENSORS on this car. These mothers are very difficult to work on. But gratifing when one is able to fix them.
THANKS

There has to be a way to unplug the O2 sensors, because they have to be replaced from time to time. I wonder what blew the fuse?

They can, I've changed them a little over a year ago. The plugs are located in a space behind the engine just very little room.

Well drove around 80 miles today, numerous stops. Towards the end of the day go to start the car and the Check Engine light comes on again. So it may seem the problem persists. One thing I should note is that its a 20A fuse originally and I replaced with 15A. It looks like I'll have some checking to do.

Test it as per the instructions above. You'll get it.

why put a 15 in where a 20 should be?

I didn't have a 20A available at home. Shall replace today and see.

It's safer than putting a 30 where a 20 should be :)

true enough

20A fuse installed, drove all day, so far so good. Shall see what happens for remainder of week than check on the weekend.

Thanks

so the 02's have been changed? Did you get direct fit or did you buy those generic pieces of garbage that you have to cut the plugs off and rewire? That could be your issue if they were in fact what you put in last year!