P0603 — Internal Control Module Keep Alive Memory Error

What it means

The keep alive memory, which holds fuel trims, idle and throttle adaptations, misfire counters and readiness monitors between ignition cycles, has lost its content or failed its integrity test. This memory is powered continuously from a permanent battery feed. After the loss the engine relearns everything from base values, which gives a rough idle, uneven running and worse fuel economy for the first tens of kilometres, and readiness monitors are reset so the vehicle will not pass an emissions test.

Likely causes (most common first)

1. Interrupted permanent battery feed to the ECU: blown fuse, corroded pin or a broken wire
2. Battery disconnected, replaced or fully discharged, or a jump start with reversed polarity
3. Voltage drop across a poor ECU ground point on the body
4. Degraded EEPROM cells inside the module after many write cycles
5. Moisture and corrosion inside the ECU housing or its connector
6. Internal processor or memory chip failure in the control module

Can you drive with it?

Short-term driving is possible while the adaptations relearn, but if the code returns after clearing the power supply and the module must be checked.