2000 Grand Marquis P0720 & P0722 Codes

I am currently working on a 2000 Mercury Grand Marquis LS with 142000 miles. It originally came to me with a P0720, P0722. & P0503 DTCs. I checked continuity on the wiring harness from both the VSS and OSS all the way to the pins on the PCM and have less than1 ohm on all wires. I did notice that I would get a random M ohm readings when accidentally probing the wrong PCM connector terminal, but I had not unplugged all sensors that the harness was connected to so I'm not sure if that is normal or does this indicate a possible short somewhere in the harness? Following the wiring diagram I have also removed and cleaned all pertinent grounds. After initially clearing the codes now I only get P0720 & P0722 returning after driving the car a few miles each time I clear the codes.
Another symptom is the O/D off light will flash  occasionally for a short time, but will usually go off after getting up to highway speeds. When researching this I find that it can be caused from an overheating transmission, but  the light will come on even when the car has not been driven very far and has not warmed up yet.
Just to make sure, I have replaced both the OSS an VSS which didn't help. I hate throwing parts at something just to try to fix the problem and in this case it did not help. Fortunately they were  not real expensive.
Any help would be greatly appreciated,
 

I posted a repair question about DTC P0720 & P0722 on a 2000 Grand Marquis 8 days ago and never received a reply. I was under the impression that the forum was monitored by 5 qualified hand picked techs that would get back to me. It makes is less appealing to subscribe as I was considering doing.

What testing have you done so far?

Both codes are for insufficient input and loss of OSS Speed Sensor signal.

 

has the trany been swaped , cause my understanding is later trans come with two sensor and will work. GOOD LUCK

I would get a random M ohm readings when accidentally probing the wrong PCM connector terminal, but I had not unplugged all sensors that the harness was connected to so I'm not sure if that is normal or does this indicate a possible short somewhere in the harness?

Without knowing, exactly, what you were probing, it is hard to answer that question.

Do you have an OSS pid, in scan data? The OSS is a sensor that will generate its own AC voltage. It should be an AC sine-wave that increases in frequency and amplitude with speed. The preferred way to view this signal, other than a scan data pid, would be an oscilloscope. A DVOM can be used, as well. 

According to SI, OSS resistance should be 334-759 Ω.  Disconnect the sensor and check the resistance between the two pins of the sensor. 

 

The Gray/Red wire is a sensor ground that is shared with many other sensors. The Dark Blue/Yellow wire is the OSS signal.

With the sensor disconnected, disconnect the PCM connector. Check for continuity between the Dark Blue/Yellow signal wire and the Gray/Red sensor ground wire. There should be none.

Check the continuity of the  Dark Blue/Yellow signal wire, from end-to-end (PCM connector--->Sensor connector). Resistance should be less than 4Ω. If you have no continuity, OL MΩ, then the wire is broken. If resistance is less than 4 Ω, then you can conclude that the wire is not completely broken, however, it does not mean that the wire is good. You should verify that the wire can carry a load by using it to power an incandescent test-light. Test the Gray/Red ground wire the same way.

Leave the PCM disconnected, but re-connect the OSS. Check continuity between PCM connector Pins 84 and 91. Resistance should be within the specs of the OSS sensor, 334-759 Ω.
 

Thank you for getting back to me. I have actually ran a new wire from the OSS to the PCM to replace the blue/yellow wire. Also when probing all sensor connectors that share the Gray/red wire for opens and /or shorts I found that the left downstream O2 sensor had a slight short to ground.(5M ohm). I didn’t think it should have that reading. After replacing that O2 sensor the problem went away for a much longer time.
I will follow up with your additional information.
Thanks again