2004 f150 4.6 4x4. Recently had a bad transmission leak.I pulled the trans. replaced the pump seals and bushings all back together and leaks seemed fixed.However now my service engine light has come on and my catalytic converter glows cherry red after a short drive.Which I have read is most commonly from raw fuel entering the exhaust and burning in the cat. I bought a cheap code reader (no live data) and came up with po174 code system to lean bank 2. I checked the obvious first made sure o2 sensors were plugged in all seamed good. Checked for obvious vacuum leaks.Then moved to the Mass air flow sensor. I watched a youtube video that said to check it by unplugging the wiring harness while the engine is running.If engine dies it's good if it doesn't it's bad.Mine still ran after unplugging it.So I dug a little deeper and watched another video on how to test it with a multimeter .I got my multimeter out but didn't seam to work proper so I used a test light.I did test 1 check the red power wire light went on. Went to test 2 check the black ground wire light went on great. Went to test 3 check second Ground Circuit of the MAF sensor (this is known as the MAF RTN circuit) Tan wire No light came on. This ground is provided by the PCM (Powertrain Control Module=Fuel Injection Computer) internally. Any idea how to fix this ? I am not a very good mechanic but I am a lot worse electrician. Thanks Youngblood

 

Here is the link to the 2nd test I used but used a test light. https://easyautodiagnostics.com/ford...sensor-tests-3

 

If your MAF is bad, you should get a MAF code from the computer shouldn't you? Talking about grounds, I do know there is usually a ground on one of the bellhousing bolts that goes up and grounds the cab sheetmetal on the older trucks. Not sure if yours has this, but I would look around up there very good and see if you did indeed leave a ground wire off that goes underneath one of the bellhousing bolts.

 

Thanks for the suggestion about the ground on the bellhousing bolt. Dave the only bolt that I remember having anything on it was the bolt behind the fuel rail ( one of two bolts that took 3ft. worth of extensions to get to) It had a bracket to hold the fuel lines it is possible it had a ground on it all so.I will see if I can find a jail house mirror and a good light to see up in there. lol The po174 code is the only code I have gotten but I have had the battery disconnected and haven't driven very far with the cat getting that hot I am afraid to run it for long. Thanks Youngblood

 

All of the sensor and signal returns should show zero ohms to chassis ground. There's a jumper internal to the PCM that makes that final connection between the three grounding circuits (chassis, power, signal).

Check the continuity of that MAF_RTN wire back to it's PCM pin and make sure it's not open. Check the external grounding straps near the PCM and also as Franklin2 suggested.

 

Cleaned every ground I could find. Couldn't see if I missed a ground on the bellhousing like Franklin mentioned but ran a jumper to my battery. Maf RTN test still came back negative. Checked continuity between MAF RTN wire at the plug for MAF sensor to it's pin on the PCM I had continuity.Does that mean most likely the problem is in the PCM ? Thanks Youngbood

 

I also replaced my leaky manifold on the passenger side. I snapped a bolt off and used my flux core welder to extract it. Is it possible I damaged the PCM ? I undid the positive lead for the battery but didn't undo the negative.

 

Definitely possible, especially if the welder decided to use a ground inside the PCM instead of directly back to the ground on the welder. I can't explain why or how it happens, but have seen many times vehicles would not run at all after some minor welding was done to them.

 

If you want to find and inspect it, the MAF circuit ground bolts down behind the battery in the right rear corner of the engine compartment.

 

Checked that ground look good and had good continuity to the neg.on the battery.Ran the truck for awhile because I figured I had more codes that were not showing up from having the battery disconnected and not driving very long.Code 174 came up again along with 300 305 306 and 316 and could smell and here the cat very hot . Would a non working Maf cause all this or am I chasing the wrong demon ?

 

The other day while I was checking the continuity of the Maf RTN wire to the PCM connector pin I noticed a couple of wires right next to the connector that looked to have a narrow piece of black tape on them is that normal ? I also saw on the connector written in black marker 9/11 that really alarmed me lol. Is it common for there to be a date on the connector from the factory or more likely it was serviced since ? Thanks Youngblood

 

Well like I sad in my 1st post I am not a very good mechanic and a much worse electrician. The test for the MAF circuit in my second link that I used I think is wrong.I bought a proper working multimeter and did the test again.The 3rd test MAF RTN circuit I had 12 volts before I turned the key on (because that is going to chassis ground right?) then when I turned the key on I had 5 volts ( because that is signal ground right ? ) In that test he said you should have 12 volts with the key on which is wrong.Thanks for trying to steer me right projectSHO89 but I didn't even know what a signal ground was until I did a bunch of reading.That stupid test cost me days of my life I will never get back.I think I am ready to dig deep and take my truck to a real mechanic.Thanks for your help guys.

 

You've actually been using the wrong diagnostic procedure for your vehicle, that one you've been using is for an older electronics system than yours. Starting in 2004 with the engines in the NBS, the MAF and IAT were integrated into a single module and it's now a 6-wire sensor.

 

My truck is a 04 Heritage which I believe is old style.It has a 4 wire sensor.

 

It would have been appropriate to explicitly identify that up front...