The "check engine" light on my '98 4.2 came on recently. They scanned the codes for me at Autozone and informed me it was likely a bad oxy sensor. To make a very long story as short as possible, I ended up replacing all 4 sensors (2 pre-cat, 2 post-).
I figured they were about due anyway since I am now at 87,500 miles, and the gas mileage of late has been worse than ever (12-13 mpg). But after installing them and clearing the codes, the light is back on again. One of the Autozone guys told me the problem could be that the dealer-supplied parts are slightly different, though this seems unlikely as the new sensors are Bosch just like the originals. Perhaps my problem is not oxy sensors at all??? Can anyone out there in Ford truckland shed some light on this topic? Thanks a lot, Andy

 

Hey, how ya doin? I know better if your trucks light was out.
What codes do you show? You could have a bad new sensor. I dont buy the parts not the same idea. Make sure all the sensors are plugged in good. Have another store scan your truck if need be. Let us know

 

Thanks Dave, good suggestions. To answer your question, originally I believe there was only one code stored, "Bank 2 lean". (Lean? I thought when an oxy sensor fails the ECU normally defaults to a richer setting?)....which would explain the crummy gas mileage I am getting....?
At any rate, after replacing both sensors in Bank 2, and erasing the codes, after 10 minutes or so of driving the light would come back on and stay on. So I replaced the ones in Bank 1 also and cleared the codes again . Sure enough, after a few minutes the light came back on.
I have had the readings done at two separate Autozone facilities, and they came up with the same readings, so I don't think the scanners are defective. Also, on the most recent scan I think there were as many as 4 error codes stored, this time "Bank 1 lean" as well as Bank 2. This is getting weirder by the day!
I will try your suggestion to make sure everything is plugged in tight. But I am not holding out too much hope as I definitely heard the connectors snap together when I installed them.
Thanks again, Andy

 

Low fuel pressure due to a clogged filter, bad regulator, or pump, can cause a lean condition that will result in poor mileage. So can clogged injectors.

 

Andy-
When the injectors are told they are lean it can be caused by a number of intake side sensor malfunctions or dirty intake parts, as well. Cleaning of the sensors on your intake (air and fuel side), cleaning the air intake hose itself, and the throttle body could help you narrow it down.

The reasons your gas mileage may be dropping is because of one or more of these sensors is malfunctioning telling the computer your engine is running lean or in fact it could be something (carbon/dirt/grease) covering it up. The computer just increases the fuel injector pulse width causing it to inject longer and enriching the mix. Now you have more fuel going in than normal, so your mileage drops.

Got a manual(Chilton's) and a multimeter and usually you can check to see if you have a sensor failure after you've verified it's clean.

-Kerry

 

Thanks a lot guys, your suggestions are very helpful. I believe I may have found the problem, and I don't believe it was the oxy sensors after all! Poked around the throttle body/intake boot area and bingo! found an unattended air boot nipple with a disconnected pipe below. The end of the pipe was quite grungy, indicating it had been disconnected for quite some time.

My guess is that the resultant air leak, which occurred downstream of the air flow meter, went unrecognized by the ECU, resulting in an overly lean mixture. But the oxy sensors (all 4 of them) recognized the lean condition and tried to compensate by going full-rich (which resulted in really lousy gas mileage).

It looks pretty easy to leave the pipe off the air boot if the boot is removed for service. I have now reconnected the pipe, reset the codes and hoping a) the 'check engine' light stays off and b) gas mileage is a little better. I hope the above will be of use to someone. Thanks again for your suggestions. Andy

 

hey big 6,

can you describe in a bit more detail where & how you found the loose connection? i'm also getting codes saying both banks are lean. it was suggested here to check for a vacuum leak (the guys on this forum know everything). my problem is that i have a e150 van with the 5.4 engine, so it's almost impossible to get a good look at things. when you open the hood everything is too far back to see, when you take off the cover in the interior everything is too far forward (and right below the dash) to see very well. can you give me some ideas - did you just see a loose hose, were you able to hear it over the engine noise, etc? was it a hose going to the throttle body? the front, the back? i realize your engine setup is probably different than mine, but i'm just looking for all the help i can get. also, my van doesnt fit in my garage, and it's been cold here forever (n. illinois), so my ability to work on this thing is severely limited.
thanks for your help!

 

One of the more common things I've seen that causes a lean on both banks code is a dirty MAF sensor. It's easy to clean so I think I'd try that after I looked for vacuum or unmetered air leaks.

 

On 5.4 motors check the PCV hose system.

 

Update: Light is back on!

Response to Myke: Turns out the disconnected hose I found was not the culprit. This hose simply goes from the top of the valve cover to the air boot (upstream of the throttle body) which routes any blow-by back into the intake. Having it disconnected does not affect the mixture enough to speak of.

Since my last post I ended up taking the truck to my Ford dealer; they charged me 49.95 to read and re-set the codes. Service tech said the indicated failure was a bad driver's side front oxy sensor. He advised I return it to Autozone for a replacement, and bring it back to erase the codes. He said sometimes the auto parts stores cannot adequately erase the codes, but I'm not sure about that now... as the light popped back on not 30 minutes after leaving the shop. Guess I will replace the sensor and erase the codes again. If the light comes back on I will have to try something else.

Racer: Where is the MAF sensor located on the 4.2, and how do I clean it? .

 

The hose upstream of the throttle body CAN change the mixture as you suspected in an earlier post. It is unmetered air. Instead of all the air the engine is using going through the MAF sensor, the air going in the disconnected hose bypasses that. The MAF then doesnt get an accurate measurement of the air going in to the engine.
The MAF is inside the air filter housing. You get at it by opening up the air cleaner housing like if you were changing the air filter and look into the end that's still attached to the big hose to the engine. You'll see a piece sticking down that has 2 thin wires in it. These wires are what needs cleaning. Take some Carb-Medic carb cleaner spray and spray them til they are clean. You can also take a Q-Tip and very gently wipe them down after spraying them. Careful though....the wires break easily. if they break you have to replace the whole MAF assembly. After that, disconnect the battery cables for 10 minutes then reconnect and take it for a spin. It will take a few miles for the PCM to relearn idle etc.

 

I had the same problem. Replaced all the sensors and I still had the error....

Replace your fuel filter and see if that helps. I did that on mine and the light went away for good!

The more I researched it the more I found that a dirty fuel filter can cause the sensors to report they are faulty.

Which would you rather replace? A 12 dollar part in about 15 minutes? Or all of your sensors?

Let me know if this helps.

Chris

 

Good point Chris,
Anything that affects the fuel pressure can cause the mixture to not be correct.
Most of the time, in my experience, though a plugged fuel filter will cause more driveablity problems that just the check engine light on but you never know

 

Thanks very much Chris and Dave! This weekend I will definitely try both suggestions. Andy