P0156 Code

"02 Sensor Circuit Malfunction (Bank 2 Sensor 2)" on a 2000 XJ.

I replaced the sensor but still get the code after 3 run cycles. I also have inspected the wiring visually as best I can and didn't find any problem.

Any suggestions as to more diagnosing procedures to track down the problem?

Thanks

Bank 2 is Cyl 4-5-6, sensor 2 is the post mini-CAT. Is that the one you changed?

Here's some general tips on this code:

http://www.obd-codes.com/p0156

Yes, it is.

I moved up the line and unplugged/inspected the PCM plugs. All looked fine, but I just got back from my 5th "starting from overnight cold" drive and the code hasn't returned. It had been coming back on every 3rd drive after resetting, so my fingers are crossed that took care of it.

If not, I have a scanner arriving tomorrow to see the real time sensor readings. Next in line will be to check closely for an exhaust leak upstream of the sensor, I just replaced the head gasket so I could have cracked the manifold during that operation. After that I will test the wiring between the sensor plug and the PCM. That's really the information I was looking for, the proper procedure for testing the circuit/wiring.

Thanks.

what happens if you unplug the sensor? same code?

sidewaysstarion said:

what happens if you unplug the sensor? same code?

Click to expand...

Never thought of that. Seems like if the wiring is open that should throw the same code though a short may be different.

I'll add that to my list of things to try if the code comes back.

Thanks

A P0156 code refers to the Bank 2,2 o2 sensor or the downstream (post-cat) o2 sensor on Bank 2. It is a four wire sensor. The PCM supplies a ground circuit and a reference voltage circuit of about 0.5 volts. Also for the o2 sensor heater element there is a battery voltage supply wire and another ground circuit for that. The o2 sensor heater allows the o2 sensor to warm up faster, thus achieving closed loop in less time than it would normally take for the exhaust to warm the sensor up to operating temperature. This P0156 code indicates that the Bank 2, 2 o2 sensor is stuck low for too long or isn't active at all.

I had the same problem this spring.I had a cheap ebay codescanner that was reporting P0156. Clearing the code would work for ~1000 miles. When I hooked a nicer code scanner it said P0456 (small evap leak) and a few minutes of listening under the jeep led me to a rotten evap tube. Are you using a cheap codescanner?

Well I still haven't had the code come back, but I did try out the scanner I had ordered anyway.

The top frame are sensors 1-1 and 2-1 and the bottom is 1-2 and 2-2. Both bank 2 sensors are in green. Notice that the green lines show a pronounced lower reading than the red lines. My code was for the post cat sensor, but this shows both are really reading low on that bank. As Tim_MN pointed out, my code meant "o2 sensor is stuck low for too long or isn't active at all". The green lines look like thay aren't far from meeting that criteria and I expect the code to return at any time.

I think this rules out an electrical problem with the sensors as it looks like they are sending their respective signals just fine. When I replaced the head gasket, I just unbolted the exhaust manifold and pulled it to the side far enough for clearance. I may have caused a crack and will inspect carefully next oil change.