I have been giving some thought to what you have written.

You said you were going to check all the ground connections on the insides of the fenders. I hope you meant between them as the fenders themselves are possibly made of plastic or are isolated from the frame and engine.

The most important think is look for a ground strap that hooks the engine to frame and another one to the car body. They should be of a flat braided copper zinc coated material but any good conductor will work. Check under the ends especially the body where it is painted. Pull the bolts and clean and lubricate them.

I have a trick that I have used and will write this in for others to think about.

The one little trick that you might consider is done with your digital voltmeter. Use the low scale range like 20 volts. Auto range will work too! We want to measure the negative ground wires to the ground connection and in parallel to itself. You can even check back to the battery minus terminal for comparison using an auxiliary wire like we had talked about before. All this is for the purpose of looking for excessive resistance where ever it may hide.

I know this is about to sound crazy but I’m half way there most the time anyway.

I’ll try to explain this as best as I understand it. Electrons flow from the battery negative post through the ground wire to body or frame. They go through the component operated then back through the positive post. We think we switch on and off the positive side but electrons really don’t see that way. The fact is their blind to color and their lazy when taking trips.

They prefer the shortest routes. Shorts last only for a SHORT time. It is an over used term. It burns itself OPEN. It’s the fuses (luckily) then the wire or a component like a wound coil, light bulb etc that open.

People want to say it has quit working because “it must be a short!” Not so! It now is an open circuit. It is bad component of the circuit or the whole other half, “A” ground. It is “All” the same.

That grates me so I have to try to debunk that statement where ever I can. Sorry!

If you take your meter and put the leads “across” the post you get the reading of potential battery voltage. 12.6 fully charged at rest. No resistance except the leads which you won’t see because you have no reference to know what that is.

If you place the leads in series or the same side you should get zero plus some minor meter circuit and the leads resistance. This is true ground zero, per say!
That should like .000 to .1 part of one volt.

If you have any resistance in excess of .2 or higher then there is a less than ideal connection or some loaded circuit back to the battery.

If there should be only a good direct to ground wire connection you are looking for and you get like a .5 volt. You have some resistance! Corrosion or bad wire is most the common.

This will show up in some amount of voltage drop. This then equals a drop in performance of the intended operation i.e. a dim headlight is a visual result.

In the case of fuel injectors and sensors that use less than 5 volts and a signal (almost no current) of even less voltage or using resistance value transfer. Then this gets crucial!

The higher the voltage reading on the minus side means you are starting to place “across” to the post. Not in line with the one side. This is not what you want to see since you are looking for the most pure grounded circuit connection. A zero ground!

You can play with different resistant loads across a small battery and read the voltage changes to get the idea how much resistance it takes to get very low reading in voltage. None the less it will show up.

I thought you might use this technique and have a more fruitful outcome while doing your detective work. I would start with big ground connections i.e. battery or engine and work your way to smaller circuits. Check in parallel to the ground wire for quickness and if it is even suspicious pull it and check both sides. You’ll get the hang of it with practice.

You said you were going to check the sensors. It is the same thing except your ground side of the sensor has a car in the middle of it and the ECU point. The output of the sensor is like the battery and has to circle back. It goes by the wire or the base metals.

There could be a relay that grounds these wires upon activation of any kind of mode switch. This could be a separate timing relay or after a time out from inside the ECU. It is all possible to bring in the sensors separately. They do this on my ’78. Yours is probably similar.

To find these you are going to need a map. Hopefully it will be easier. I think that since you changed the ECU you may have shook the bush. You said it did better after you changed the ECU.

Have you tried your local libraries for manual and wiring diagrams? I have had good luck using those resources. The older the vehicle the better the chance! They can get stuff from others and some have computer "ask" sites they pay for!

Thanks for clearing up the codes. I wonder if the memory held more than one or had more areas to report.

Hope this helped and didn’t bore you or any one else.
Phil