This is my first post. I have recently replaced both 02 sensors on my 96 F150 with 302 engine. Got new Bosch sensors at Advance. I still get codes P0135 O2 Sensor Heater Malfunction bank 1 sensor 1, P0141 O2 Sensor Heater Malfunction bank 1 sensor 2,PO155 O2 Sensor Heater Malfunction Bank 2 Sensor 1 and P0171 System too Lean. I have replaced the PCV and fuel filter as well. Is the problem with the new O2 sensors or are they just doing their job and do the injectors need cleaned? Any help would be appreciated.

 

There's always the possibility that your engine is running so lean that it's beyond the ability of the system to correct for.

The first thing I would check is for a vacuum leak. Especially around the throttle body. The gasket tends to get sucked in after time.

 

and check your fuel pressure, if you don't have enough then you could get the lean condiditon.

 

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The codes said heater circuit malfunction, so I would start by checking the fuse in the fuse block under the hood. If that fuse is good, start testing to make sure the O2 sensor heater circuit is properly grounded, and that the O2 sensors are in fact getting 12V where they plug into the harness. I think if the heater malfunction is corrected that the lean mix will go away too. I understand the lean code to mean the O2 sensor reads rich (which it will do if the heater isn't working) and the computer leans out the mixture, and now it has leaned it so much that it can't lean it anymore, but the O2 sensor still reads rich.

 

I have checked the ground for each O2 sensor and they are grounded. I am not sure where the fuse block under the hood is as mentioned below. There is not a fuse for the O2 sensors under the dash, I checked. Can I run a jumper between the connectors and check to see if they are getting power? I figure I ought to check and see if these sensors are working before changing the throttle body gasket or checking fuel pressure. Is there a chance they gave me the wrong O2 sensor items and the connectors are out of sequence. The plug seemed to mate up properly. Both left and right side sensors were identical. Changing the upper throttle body gasket seems like a more difficult task on the "96 302 5.0L FI engine with 103K miles. Thanks for your responses.

 

There should be a fuse/relay box right behind the air filter I believe. It's also possible that the relay isn't working for the O2 sensors if there is one specific to the O2 sensors. The sensors themselves are probably fine.

 

I took my truck to the dealer for the cruise control recall and they ran down my O2 sensor problem using a break out box. It cost me but I wanted to know what the problem was. It diagnosed that the PCM was faulty and needed replaced. They indicated this was a rare occurance and they have only seen 2 of these failures. It will cost between $75 at the junk yard and $185 at NAPA. I told them I would install it. Until that time I will get lousy gas mileage. Thanks for all your help.

 

I had the same problem with the check engine light. Pulled the codes, P0141, P0155. I replaced all 3 sensors. Still got the same codes and check engine light still on. It said something about a fault ground. Checked all wiring, and seemed to check out good. Finally a Ford Dealer Mechanic said, if you checked out all that was mentioned on here, and they checked out ok, with out hesitation, he said it was the ECU. I replaced the ECU, and to my surprise, the check engine light went out and stayed out after resetting it. I just don't understand with all this expensive equippment there is to pin point the trouble, it does not give you the exact problem. I think it should have said, you have a bad ECU! just my 2cents.

 

I finally replaced the ECU or PCM as it is sometimes called. It was easier than I thought. I could not find a cover inside the firewall by the parking brake release so I just removed the 2 stud nuts on the seal inside the engine compartment and pulled it out and replaced it with a remanufactured one. Much better price than the dealer wanted plus I get the core refund. I had to loosen the front left inside wheel skirting in order to have enough clearance to swap them out. I noticed a differece right away in performance and will check mileage on my next fill up. Before this repair it was using so much gas with the lean O2 sensor readings that I thought I had a hole in my tank.

 

I think my ECU is on messed up in my truck also. Mine is using excessive gas, "check engine" comes on and off, and runs and idles like crap... ...most of the time. When you floor it or rap on the throttle it runs fine. But I have the added factor that it won't send any trouble codes, some times it will send a partial code sequence or say that the system is unavailible.
Any knowledge on this would be helpful.
By the way,who's got the best deals on ECU's? Is their anyway of boosting performance with something aftermarket? I guess I need to fix my original problem first; but two birds, one stone?

 

I also replaced my ECU. Check engine light went out, and there was a world of difference in performance

 

This is very, very interesting news.....

How long did it take for the Dealership guys to test your ECM? If its an hour labor or something, I think I should go get it done.

I have the constant lean codes (on both banks on an i-6) and recently have now a o2 malfunction code on one of the banks. Get terrible fuel mileage. Replaced o2 sensors months ago with Bosch (proper part no.).

 

The dealer took about an hour to diagnose. You know what that costs. They used the break out box which goes beyond normal code readers. I told them I just replaced the O2 sensors and I wanted to know why I was still getting lean bank 1 and 2 readings. They indicated it was rare to see the ECU go out and I am not sure if that is the case or not. You must make sure everything else is wired correctly before installing or you will ruin the replacement ECU. The calibration code is on the drivers inside door on my 96 model. I purchased mine from Rock Auto on the web. They all come from Cardone which remanufactures most ECUs for everybody and have a 1 year guarantee. they use original cores and repaint them. The markings on the original were slightly different from the replacement but I used the calibration code on the inside of the door to make my order decision. I don't know for sure but I believe they have made some enhancements from the original programming in the remanufactured computers from the original. I am using my small rear end gas tank and I can tell just from driving as long as I have on 1/4 tank that it has made a difference. With the price of gas I should get a return on my investment in 2-3 months.

 

Good job dude. I suppose I will need to do some more research, as I have no clue what 'calibration code' is.

Thanks for the detailed info thought, I'll be checking out Cardone