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O2 sensor malfunction? P0136

I have a '99 S70 and have just replaced both O2 sensors after getting codes P0135 and P0136. The new o2 sensors are now in the car and I am still getting P0136 as well as P0136 pd but no longer code P0135. My understanding about the difference between a “pd” or pending code and a regular DTC (diagnostic trouble code) is that a pending code indicates that the code has not occurred enough times for the code to “mature” into a full-blown DTC while a DTC is set when a fault is present for a sufficient amount of time. How can I be getting both at the same time? I would think that would be impossible.

If I clear the codes, the CEL will come on again after I’ve driven a mile or so. What could this mean? Will the CEL go out for good after 40 warm-up cycles if the o2 sensors are good? Could it be a simple problem such as a blown fuse connected to the offending sensor (Bank1 Sensor 2) that would be cheap and easy to fix (please, oh please!)? Assuming the brand new o2 sensors are good (the P0135 code went out right away but not the P0136), what else could be happening to generate this code?

Thank you much!

When I had open 02 circuit codes for all 4 of the 02’s on my 97 Explorer I took a quick look at the engine controls wiring diagram. Sure enough there was a fuse to be checked. The fuse was blown & the new fuse did not blow. Problem solved.

Since both of yours are also open circuit codes, you need to look at the wiring diagram. Or look in your owners manual. Is there a listing for a HEGO system fuse?

First of all, there is a lot of confusion about O2 sensor problem, but this one is almost certainly an actual problem with the downstream sensor. Could be a defective sensor. Could be a defective connector. Could be failure to seat the connector properly. Could be a fuse, but I’d expect that to take out both sensors. In theory, it could be a large hole in the exhaust, but I think you’d hear that.

If you changed the sensor yourself, try removing and reinserting the connector a couple of times. If that doesn’t do it, return the part (and yes, I know it’s a pain to remove). If someone else replaced the sensor, take the car back to them and tell them that their #$@*# downstream sensor doesn’t work. Be aware however, that they probably don’t want to deal with this any more than you do, and there is a real chance that you will get a ration of creative nonsense about your using bad gas or angering gypsies rather than their fixing the problem.