P0600, Limp mode, ER HC, ER SS

Do not overthink the problem.

Subaru modules (ECU, TCU, VDC, BIU, etc) rarely fail. You are far more likely to be having a problem with wiring, electrical connectors or earth/ground connections.

As you mention you have communication problems you need to start with battery. battery cables, and the eight to ten ground/earth connections in the engine bay. These connections all need to be clean, tight and free of corrosion.

You need to remove all the ground/earth connections and clean the earth connector and vehicle frame where the connector bolts to. There are two earth/ground connections on the inlet manifold, one on the firewall, one from each head to the frame rail, one to each side of the engine bay and the main earth/ground cable from the battery.

Once you have all these clean you need to test the battery using a CCA tester and compare the actual CCA of the battery to the rated CCA. This will give you a pass or fail result.

All of the modules and sensors on a Subaru rely on good ground/earth connections to work properly. Bad/poor ground/earth connections equals multiple communication problems.

Seagrass

Some observations, comments, . . .

AnotherSubaruGuy said:

In park the wheels were spinning, can't recall if that's normal, . . .

Click to expand...

All four? When in Park, the front wheels should not be turning. The parking pawl in the tail section engages a gear tand pinion shaft connected to the front differential. The pawl prevents the gear from turning, and therefore the front drive from turning. Are you sure this was in Park?

. . . in drive . . . it seemed to be letting the power go to the wheels as I revved the engine like it should be when all 4 wheels are in the ground

Click to expand...

Very little torque is required to turn the wheels when they're raised. This doesn't simulate the conditions when the car is on the ground and a lot more torque is needed to move the car.

. . . when I pressed the brake the ABS kicked in . . .

Click to expand...

To be expected. When the pedal is pressed with the car raised, one wheel could stop while the others continue to turn. To the ABS, that's a skidding wheel and so the ABS kicks in to control the "skid".

When the car is being driven, and the accelerator pedal is pressed down, engine torque is increased, and there should be a corresponding acceleration of the car. If the engine revs up, without any corresponding acceleration, there is some sort of "disconnect" between the engine output and the wheels. This could be in the engine/torque converter connection, the torque converter, the transmission gear set, or the drive train to the front or rear wheels (drive shafts, differentials, axles). The converter and transmission are less likely if two transmissions with their own torque converters were installed properly and the car is exhibiting identical symptoms. All the more so if the car had the exact same symptoms when first purchased, before changing the tranny. That essentially leaves the drive trains to the front and rear wheels.

There have been cases of a CV joint in an axle failing, but not falling apart. Visually it looks fine. Moreover, if one twists the two ends of the axle by hand, it might seem intact because of residual friction in the joint. But when that twisting involves higher torque, the joint begins to slip, and torque transfer is lost. The part that is connected to the differential spins freely, and that reduces the torque available to other wheels. The engine revs but the car doesn't accelerate. Similar symptoms could be experienced if an axle has pulled out of the differential. A movement of about 1/4" is enough to disengage the splines on the axle from the splines in the differential side gear. The axle appears to be in place (not hanging down), but it's not connected inside. I'm not saying these are your problem, but the symptoms would be similar to what you experienced, especially most recently when you "gave it some good gas on our gravel driveway. . . "

I left the battery disconnected overnight while I cleaned the grounds and didn't have any improvement on start up.

It was in Park on the lift, but it was only the rear wheels spinning.

The weird thing is the throttle response felt normal with the wheels off the ground, then when I lowered it down and drove it again it felt normal like the test drive after installing this current tranny. After I caused TC/VDC to kick, it went back to little to no power going to the wheels unless I gun it from a dig, then it will spin the tires.

Not to sound like a smart guy, but these cars are all I work on and I don't think I've seen the ABS kick in like that before. I mean I really had to fight with that pedal to stop the wheels.

When I first purchased the car it actually drove pretty normally but would slip under hard acceleration and kinda hunt for gears, and it had a Christmas tree for an instrument cluster. But it was not doing what it's doing now.

With it being intermittent like it is I don't feel like it's a mechanical part failing, but I could be wrong. When I press the gas and the brake at same time I can feel it build torque, and like I said it will spin tires from a dig but bog out.

I really do appreciate all of your guys' help and time