Hello all,

Every once in a while (seems like when it rains.. and it rains alot on the wet coast of Canada) my 1997 F-150 with the 4.2 V-6 (not the original engine-a Ford Quality Renewal engine) throws on the check engine light. Check the code and PO 135 - O2 sensor heater circuit malfunction (Bank 1, sensor 1) and PO 155 (O2 sensor heater circuit malfunction (Bank 2, sensor 1) comes up. Ignore the light for a few trips and it turns itself off.

Now the questions...
Is the sensor heater a part of the O2 sensor?

Need I worry about this since the check engine light resets itself when things dry out?

Is there a way of sealing the sensor better?

Thanks for your help!

Cheers,
Roy in Mission, B.C.
Canada, eh?

 

It is my understanding, that the O2 sensor itself is heated by battery voltage, in addition to the exhaust gases. This is to aquire the operating temp of the sensor faster (500deg.) The faster the sensor gets to temp, the sooner the computer is operating in closed loop mode, and better regulating the emissions. If you are getting this on both banks, then there may be something more than just a faulty sensor. Are your plugs clean?? If not, the engine may be running too cold, and not heating things up properly. The code is stored indefinitely in your computer, so you can pull that code out at any time.It has to be reset by disconnecting your battery for a few minutes, or by a diagnostic tool or computer. If you know it's directly related to rain, then it's gotta be in one central location, cause the chances of both sensor's getting water in them are kinda slim. I would get a Haynes manual and trace the power circuit to the O2 sensors and see if you can find a break or a leaky connection.

 

Thomas is right on track.
The O2 sensors are heated for about the first 3 seconds after start-up. 12V is appled to the O2 sensor heater circuit. (Yes, the heater is part of the sensor.)
After that, the heater voltage is turned off.
You can check the operation of the O2 sensors but I don't know the specific pins/wires to get on to measure. Seems like the voltage across the sensor will vary from about .1V to .9 V when the O2 sensor is operating properly and you vary the engine RPM.
I would try and find a Haynes manual. They have a section on checking/testing the O2 sensors. It would be a good investment I'd think.