CEL 12, 21, and 55 on '94 Jeep Grand Cherokee

Question

So, I've pulled the codes from my car based on the helpful information provided here:

Check Engine Light on '94 Jeep Grand Cherokee

Now, I need just a little help understanding what the codes actually mean. The translations below are taken directly from the other thread.

12 memory to controller has been cleared within 50-100 engine starts

What particular "memory" is this referring to, and how would it have been cleared? I imagine loss of battery power would be one cause. Is this also the same as the engine code memory? Does that memory get cleared each time it's read?

21 oxygen sensor signal, neither rich or lean detected

I presume this is pretty straight-forward: Check/replace O² sensors. Although it could sort of be construed to mean that the air/fuel mix is dead-on, I'm sure that's not very likely.

55 end of message

I'm guessing I should see this every time I pull codes? Sort of like this message from the MCP:

Note: This is for a '94 Jeep Grand Cherokee, 4WD, 4.0L in-line 6 cylinder engine.

Answer 1

I found this:

http://www.justanswer.com/chrysler/1l50u-does-dtc-12-memory-controller-cleared-within.html

Which seems to confirm that 12 just means the battery was disconnected and that you should always see 55.

21 should not come up unless there is a problem, because the sensor should be able to detect rich or lean. So check/replace O2 is the correct course of action.