Repeat O2 Sensor Failure

I have a 2004 Ford Escape that seems to have developed an appetite for one of its O2 sensors, about 1 a month for the past 3 months. Specifically Bank 1 Sensor 2. That is this the downstream sensor for cylinders closest to the firewall. I have replaced the sensor twice and the last sensor just popped the code to let me know it’s passed on. The code is P0136 is the code that has popped every time. The live data shows a flat 0.0 off of the sensor.

So before I replace another sensor, does anyone have an idea as to what would cause the same sensor to die 3 times in a 3 month time period?

The code indicates an O2 sensor CIRCUIT problem. Not an O2 sensor itself problem. Have you checked the connector/wires to the O2 sensor?


No not yet, this time. I certainly did in the past with no luck. The past two times changing the sensor provided my only luck. Hopefully, I’ll have time to play with it tomorrow.

Are you using anti-seize and if so, is it sensor safe?

I did not use additional anti-seize. The sensor (Bosch) came with anti-sieze pre applied.

The factory service manual will say how to test the sensor with a basic DVM (they start at ~ $20) by measuring resistance between certain pins and/or the case.
Drop by a dealership and the right person might let you peruse the manual.

Hmmm, any chance you’ve got a coolant leak into the combustion chamber(s)? Coolant level dropping?

I haven’t noticed a drop, but i’ll look again tonight.

Excess oil in the exhaust can also cause 02 sensor failure. You might also check ebay for an 02 sensor, I’ve bought a few there for less than half the price the local parts stores want.

The problem isn’t caused by coolant or oil in the exhaust. O2 sensor 2 is the O2 sensor located after the catalytic converter. If there where coolant or oil in the exhaust it would also cause problems with O2 sensor 1 which is located before the catalytic converter.


Good point tester, I missed that detail in the OP.

Thank you everyone for the thpughts and responses so far. I did some research and thoght the catalytic converter might be at fault. So the Escape spent yesterday at the Ford dealership to try and cash in on the federal emissions warranty. Well all Ford did yesterday is put in a Ford O2 sensor. It went bad at 8:30 this morning. Clearly I still have a problem. Anyone know how to test the emissions wiring.

The Federal emissions warranty is for 8 years/80k miles. Your vehicle is an '04 and the odds are that it was sold or put into service (meaning as a dealer demo, etc) back in '03. Even if the mileage is under 80 the vehicle could be out due to time constraints.

You may still have a converter problem and it sounds like the Ford dealer was making a WAG too.

The warranty is still good. According to Ford it entered service in March 2004 which gives me 5 months. Thr miles wont be the problem.

Thanks for clarifying that. You’re still good to go on the warranty.

My opinion, still, is that the dealer was probably making a WAG. Converter and O2 sensor problems are often misdiagnosed and the competency of the diagnosis will vary greatly depending on the person doing the testing.

I’d hit them right back up come Monday morning and be sure all of this is documented in writing with copies of repair orders, etc.
In case this is something that drags out until after the warranty has died this docmentation can help greatly during a Good Will warranty request.

I would have to wonder if something within the ECU is causing this trouble. It would be interesting to compare the voltage and resistance readings at the input to the ECU. It may show up a problem.


Anyone know how to test the emissions wiring?

I would suspect a temporary short in the wiring harness between the ECM and the sensor. With the engine off, pull the harness and test for electrical connectivity in the harness between the wire carrying the sensor output voltage to the ECM and engine ground. If connectivity exists, you have a short. (You’d have to know which of the four harness wires carry the output signal, but that is easy to determine.)

You can test the O2 sensor itself by pulling the harness and connecting a scope or DVM to the sensor output. Start the engine, let it run at 3000 rpm in Park or Neutral until the radiator fan turns on, then let it it idle. This should heat the sensor sufficiently to produce a signal. The signal should oscillate a few tenths of a volt centered at five volts or so, but the oscillations should not be as sharp as the signal from the primary sensor. (If the primary and secondary O2 sensors track each other, you have a dead cat.)

Mechaniker: You’ll need a scope for that, right (the oscillating signal when running)? A regular multi-meter is far too slow to show that kind of variation in a signal. It’s unlikely the OP has one.