I have a 1998 4x4 5.4L Expi. The codes p0135 and p0155 keep showing up even after everything has been replaced or checked. New intake manifold gaskets, new ford front o2's, new Bosch rear o2's, new cats, new muffler, new PCM. All wiring to sensors and PCM checked good. I have totally run out of ideas. The dealer suggests replacing the rear o2's with ford ones, other than that ???????????????????????? Suv seems to run ok but power is down from what it should be. Milage is 9-10 mpg around town. Please Help!!!! Also the suv has cold air intake, cat back exhaust and JBL shorty headers. I have a superchips tuner and a BBK throttle body I want to try but not until the codes clear up.

 

These are heater circuit malfunction codes for bank 1 sensor 1 (0135) and bank 2 sensor 2 (0155). Since you say that you have replaced all O2's and are still getting the same codes, then it sounds like you have a wiring problem. Check your connectors and cables on each sensor wiring harness. Make sure you have no wires loose at the connectors. Also, check where they connect at the PCM (I am not sure, but a search here will tell you, it was just discussed recently) and make sure all wires are connected there.

With the sensors not heating up, it would make sense that you are getting a power loss and bad MPG, as their readings are not correct. Also, Bosch makes FoMoCo O2 sensors, so if you got the specific ones for the rear, then your ok. If you got the universal set, well............

 

I had the same thing not long ago. I replaced a bad rear O2 due to a code. I replaced it with a new Bosch with the same part number as original. After replacing it I started getting the bad heater circuit codes like you. Turned out the new O2 sensor I put in was bad. Replaced it and all is fine. If I got 1 bad one I suppose it's possible you could've gotten 2.

 

I was wrong on one item I mentioned. Of the front 2 sensors I replaced them both with aftermarket ones then the dealer then replaced one with a ford sensor. All wiring was checked, at the sensor and at the PCM, it was all fine. Maybe I need to replace both front sensors again? This is getting old and expensive! Quality control on o2 sensors suck! Thanks!

 

I just wanted to let everybody knoe I replaced both front o2's and the codes went away. Thanks!