I have a 01 f150 7700 with a 5.4L and its throwing codes po135,po155, po174, and po171. Here is the history, last year had same code and found it was a bad ELLBOW on pcv hose, replaced and that fixed it, I also had a crack on egr valve and I replaced that too. This Jan truck was running bad in the morning and I traced it to a bad idle air controller, replaced it. I got a code and guy said I had a bad pcv valve so I replaced it. About 3 weeks ago check engine light came back on with the 4 codes listed above, I replaced both front o2 sensors, 250,000 mile so I thought that was it but came back same code, I took it to a local mechanic and he said that the intake gaskets need replaced, but he call me later and said I had no leaks ont the intake and that he did find another vac leak. He didn't charge me anything and sent me on my way. I drove the truck about 50 miles and same codes back, so to recap I have cleaned mad sensor, replaced both front o2 sensors, a new egr valve, a new idle air control, pcv valve, pcv hose and ELLBOW, don't know what to look at next. Any clues? Thank in advance

Jim

 

Your report is yet another example of incompetence.
LOOK at the codes!
Both 135 and 155 codes tells you the Heater 'circuits' are both open for each side's front Ox Sensors.
Did you forget to plug them up or is the harness in trouble?
Very simple problem.
You changed the sensors for naught.
The 171/174 is still an air leak somewhere.
It's not the sensors. If they stopped working you would get codes 172/175 for too rich.
This can be verified by looking at the long term fuel trims with a Scanner.
When they are sifted more than 25%they cause the codes to be set.
Seems like a little competence is lacking everywhere.
Sorry to hit this so hard but someone has to understand it if your going to get your issue solved.
Nothing about this is very difficult.
Good luck.

 

I've double checked the connections at both 02 sensors, I understand that I didn't need to change 02 sensors now and I'm not an auto mechanic and just jumped the gun because of lack of knowledge But at this point I just trying to solve the issue. As I research this problem I've found that it could be a lot more than a vac leak. I'm still looking for a starting point, I don't know if mechanic was correct and intake gasket is bad or mass air flow sensor bad or weak fuel pump or a bad electrical connection, or is there something else causing this problem, the truck seems to be running ok, I guess I need to go to another mechanic

 

If you still have the codes, the issues are still there.
The computer system never makes a mistake.
It knows the cause of these codes because the computer measures the current drawn by those heaters. That current is absent.
All takes is a voltmeter check to see it, then find the open in the harness.
.
The heaters are necessary for proper operation of the sensors during cold starts and to help keep them at their operating temperature of 600 degrees.
.
The 171/174 codes are shifted fuel tables due to an air leak.
This can be verified with a Scanner.
A good shop should know all this or they are not competent.
.
You needed to hear all this because your not getting anywhere with your problem.
This is the help you needed.
Now you need some one to execute the repairs.
The intent of the reply is that you asked for needed help and
for others to learn from your predicament when they search for similar problems.
It had to be pointed out how incompetent the results you have so far, have been, as part of the help, because it has became part of the problem or you would not have come to ask for help.
There is no more to add at this time unless you have some other issue.
Grab the Bull by the Horns and make him listen.
That means there is no Bull here.
Good luck.

 

Thank you so much. My brain was dead. I'll start checking my connections and voltages at o2 sensor first and see what I can come up with. I got focused on the lean condition of the 171 and 174 code and really wasn't looking and the 135 and 155 code, if I can find what's causing those code I might have the other codes solved too. Thanks I'll look at it more this weekend and I'll let you know what I find. Thanks bluegrass 7