P0603: Internal Control Module Keep Alive Memory Error

What P0603 tells you, and what it does not

The keep alive memory that stores learned values like idle trim and shift adaptations lost power or became corrupted, so the computer has to relearn those values from scratch.

The code sits in the engine computer learned memory area and is reported by the comprehensive component monitor. It names the circuit or the condition where the computer saw a reading outside its expected range. It does not name a failed part, and several genuinely different faults set this same code, which is the whole reason the causes below are ordered rather than listed.

What people usually notice

• Rough idle for a short time after the code sets
• Shifts feel different than usual temporarily
• Minor drivability change that improves on its own
• Slightly lower fuel economy for a short time

What causes P0603?

The causes below are ordered by how often they turn out to be the answer, cheapest and most common first. That order matters more than the list itself: the expensive component named in the code text is very rarely the first thing to replace, and working down from the top is what stops a engine computer learned memory fault becoming an expensive guess.

Battery disconnected or died recently is the one to rule out before anything else. The most common and expected cause of a keep alive memory loss.

Working out which cause it is

Start with freeze frame. There is no meaningful freeze frame, check whether the battery was recently disconnected or replaced. That snapshot is taken at the instant the code set, and it is the single most useful piece of evidence you have, which is also why clearing the code before reading it is such a costly habit.

Reading and clearing the code is enough to act on this one, which any OBD-II compliant reader can do. A basic code reader is enough to confirm this is a memory reset, no live data is needed since it typically clears itself as the computer relearns.

The order worth working in

• 1. Battery disconnected or died recently: The most common and expected cause of a keep alive memory loss.
• 2. Corroded battery terminal causing intermittent power loss: A cheap fix that also prevents this from recurring.
• 3. Blown fuse feeding keep alive power: A lost supply voltage clears this memory the same way a disconnected battery does.
• 4. Wiring fault to the keep alive circuit: Less common, but a break in this circuit can cause repeated resets.
• 5. Internal computer fault: Least likely, but possible with an aging computer.

What P0603 means for driving the car

Expect a short period of rougher idle or slightly different shift feel while the computer relearns, this is not a safety concern.

This is a severity 1 of 4 code. It will not usually strand you, but it will fail an emissions test while it is present, and the underlying fault rarely gets cheaper by being left alone.

What does it cost to ignore P0603?

There is little cost beyond a short adjustment period as the computer relearns normal operating values.

There is also the diagnostic cost of waiting. Freeze frame data, pending code history and the readiness monitor state all describe the fault as it was when it set, and all of it is lost the moment somebody clears the code to make the lamp go out.