With gas prices through the friggin' roof, I decided to see if getting my check engine light to go away would help with the mileage. I knew I had an after-cat O2 sensor issue, but I was somewhat surprised to get four codes! I looked them up, but I still don't know what they mean. Need a little guidance on codes P0411, P0401, P0420, and P0174. Think that's enough? What can I do to fix these? The truck is a '96, 300 I6, ,2wd, 5-spd, with 150k miles.

 

P0401 - EGR insufficient flow -- Check your vacuum lines. Are any broken or missing?

P0411 - Secondary air injection incorrect flow -- see above

A bad vacuum leak could potentially cause all of these codes.

P0420 -- Catalytic converter efficiency below threshold
P0174 -- Fuel system trim too lean

A failing front oxygen sensor could cause both P0420 and P0174.

Check your fuel pressure. If too high or too low, you could get P0174.

P0420 was intended to flag failure of the catalytic converter. The computer compares what is happening at the front O2 sensor with the back O2 sensor. If they are the same, the converter is not reacting the NOx with the HC/CO. But it is common for bad oxygen sensors to cause this code to flag falsely.

Once you clear codes, it takes a long time for P0420 to come back (or not come back). The computer waits for the completion of an OBD-II "drive cycle" before flagging the code.

You should get P0174 cleared up before you consider replacing the catalytic converter.


You should get P0174 to go away

 

Ok, so where is this oxygen sensor? The one guy who knows something at the local AutoZone said this code may be caused by the after-cat O2 sensor. Where is the "front oxygen sensor" you're talking about? In the exhaust? Checking fuel pressure should be no problem. Is there a handy diagram or cheat sheet on how many and where all the vacuum hoses are?

 

Ok, today I drove about 100 miles, in all types of driving, and the check engine light hasn't come back on yet. How long is this OBD-II drive cycle? I'm sure it'll come back on, but what does WHEN it comes on tell me?

 

Follow the exhaust system starting at the cylinder ports. Look for a 3 inch long "probe" stuck into the side, with 4 wires comiing out of it. The first one you come to is the front oxygen sensor. This one can have a major impact on engine efficiency. Now crawl under the truck. You should see another oxygen sensor in the exhaust system a bit downstream from the catalytic converter. This is the after-CAT O2 sensor. This latter one is only used for OBD-II diagnostic purposes, the truck will not run better and will not get better gas mileage if you replace it. You may be able to get rid of the code P0420 and the check engine light by replacing it, however.

There should be a vacuum line diagram under the hood. Or you can get an alldatadiy subscription. Look at my gallery for an example of what you can get from alldatadiy -- unfortunately this is for a 302 and an earlier model year. Unless someone has messed with the vacuum lines, you are looking for mechanical damage and so the specific routing doesn't matter -- just keep everything the way it is but look for cracks, hoses disconnected, etc.

It can take as much as two weeks for P0420 to come back. What this is telling you is that the problem is not too severe, but persistant enough to consider replacing the oxygen sensors if they are high miles. The 1996 model year vehicles were Ford's first try at this, and the sensing of code P0420 was a little too touchy for some vehicles. At least for the 4.0 V6, there are TSBs that say to get the computer's calibration updated to the latest version before spending additional shop labor on trying to fix P0420. Since the computer calibration is in flash EPROM, this involves no more than connecting it to one of those fancy Rotunda testers and downloading the new programming.

 

Thank you so much for all your help. The light came back on today, by the way, but I haven't had a chance to plug in the reader yet. So, to sum, I need to check all my vacuum lines, and maybe spend a little money on a front oxygen sensor. I think I can handle that. I really hate giving those dealership garages any money, and extra at the gas pump sucks too.