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Glenn, Technician/Trainer
Category: Ford
Satisfied Customers: 606
Experience:  28 years experience, Interprovincially Licenced Motor Vehicle Mechanic and Instructor
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1996 F150 V6 4.9L Check engine light on. Error code PO135,

Customer Question

1996 F150 V6 4.9L Check engine light on. Error code PO135, 141, 155. Probable causes Oxygen sensor defective / Poor cnnection at oxygen sensor / check circuit-fuse. Replaced all 3 sensors reset error code and check engine light returned. Checked voltages and all looked good. What should I do now?
Submitted: 8 years ago.
Category: Ford
Expert:  Glenn replied 8 years ago.

Hello , my name is Glenn. Welcome to JustAnswer.


P0135 and P0141 both relate to the oxygen sensor heater circuit.


Do you own a testlight ? Would you like help diagnosing this problem ?



Customer: replied 8 years ago.
Relist: I still need help.
Customer: replied 8 years ago.
I do not have a test light but could get one...if needed. I'm currently checking the resistance on all the wires.
Expert:  Ron replied 8 years ago.



Thank you for asking your question on JustAnswer.


The common cause for the issue your having is a bad PCM or a ground issue. Lets start here.

Check Vehicle Power (VPWR) voltage on the Red (R) wire at the Oxygen (O2) sensors.

Check for battery voltage at PCM pins 93 (Red/White (RD/WT) wire), 94 (Yellow/Light Blue (Y/LB) wire) and 95 (White/Black (WT/BK) wire) with the key on and all O2 sensors connected.

Check PCM ground activation of the oxygen heater elements after starting the engine. When the engine is started, the PCM should ground activate the circuits at pins 93 and 94. If the PCM does not ground these circuits, replace it.


Expert:  Glenn replied 8 years ago.

Hi Rodney,


What Ron has told you here is that the computer has likely lost the ability to "turn on" the oxygen sensor heaters on your truck.


I have seen this numerous times. The heater is controlled through the computer on this truck to allow it to continually test the operation of your oxygen sensor heater circuits.


The reason I asked you to get a testlight is that I was going to instruct you on a way to test this circuit.


Do you still need further explanation here or did you understand what Ron told you?

Customer: replied 8 years ago.

I have cheched those wires with the sensor disconnected and the red/white, yellow/lt blue, white/black all start with 12 volts and then go to zero. As mentioned above. Do the sensors have to be connected? All wires ohm check good to the fire wall. Not sure where the PCM is located, will need to get a better manual. Currently I'm useing Haynes, which is not all that helpfull. As for the Test light is there something I can do with it as opposed to using a voltmeter?

Also is the PCM a possible problem or is it just a connector block? It it is a connector block then am I looking at a bad computer?

Customer: replied 8 years ago.
Disreguard the PCM and computer portion, I now know they are one in the same.
Expert:  Ron replied 8 years ago.
Vehicle Power (VPWR) voltage on the Red (R) wire at the Oxygen (O2) sensors with the sensors unplugged because your checking for heater circuit voltage.

Check for battery voltage at PCM pins 93 (Red/White (RD/WT) wire), 94 (Yellow/Light Blue (Y/LB) wire) and 95 (White/Black (WT/BK) wire) with the key on and all O2 sensors connected. If both of these circuits check ok then its likely your PCM isnt allowing the HO2S to switch and is likely bad.
Expert:  Glenn replied 8 years ago.

Hi Rodney.


To add to what Ron has contributed here, I am providing a wiring diagram




The powertrain control module connector is circled on the next diagram. You can only see the connector on the firewall. The pcm is inside the vehicle on the other side of the firewall.





This is what the pcm conector looks like.




Just to add to Ron's answer, you could also test the o2 sensor heater circuits for complete failure by using a testlight


Hook up your testlight to BATTERY +. Touch the negative terminal at the battery to make sure your testlight works.


Turn the key off. Unplug the o2 sensors. Start the engine up.


Take your testlight (still connected to BATTERY + ) and touch it to the terminals for the oxygen sensor heater control wires going to the computer . Y/LB, R/W, W/BK.

If the testlight does NOT light up, there is no ground circuit for the heaters. You most likely have a damaged computer. Check the continuity of the wires back to the computer (with the computer disconnected) and if the harness pins look good. Order a new computer.


Note: if the testlight lights up, you may still have a problem with the computer. The o2 sensor heaters can consume 2 amps or more when starting up. The testlight is 1/4Amp or less.


I hope this additional information is of assistance.



Expert:  Ron replied 8 years ago.
I agree with waht Glen is telling you.
Expert:  Glenn replied 8 years ago.



Just following up to see how you have made out with the tests we suggested.