Keyboard and trackpad not working, system unresponsive

MacBook Pro Unresponsive - 4SNS/1/COOOOOO8:TsOP—124

I have a MacBook Pro 8,1 which has been working well for years after some initial logic board problems. The optical drive was removed and replaced with an SSD years ago. It is set up as a fusion drive with the main HDD.

Now it is doing the following:

Keyboard and trackpad are completely unresponsive. No light for caps lock.

Fans are running full speed continuously, regardless of how we are booted, or what account we're using, even target disk mode.

Kernel Task uses full CPU, making the system almost entirely unresponsive. I assume it thinks things might be overheating, so it's trying to save the system.

You can use it with an external keyboard and mouse, but just barely.

Cannot run SMC reset, because of the keyboard issue.

Did reset PRAM, no change.

Ran Diskwarrior which found no problems.

Booted in Target Disk Mode, and cloned the drive to an external.

Ran the AHT, which returned 4SNS/1/COOOOOO8:TsOP—124

Seems like that could be multiple sensors at Trackpad, keyboard, etc.

I have not tried wiping the drive and reinstalling OS X.

At this point, is it practical to repair this machine? I'm not confident that a top case replacement will fix the issue, as it seems like more than just the keyboard. Changing the logic board seems more expensive than is worth for a six year old machine.

But, when it works, it is adequate for our needs, and I'm cheap and hate to throw things out.

Any help is much appreciated.