OBD-II Code P0603 — What It Means & How to Fix It

What Does This Error Mean?

P0603 means the engine control module's Keep Alive Memory has an error. Keep Alive Memory (KAM) is a small amount of memory that the ECM uses to store learned values — like fuel trim corrections and idle adaptations — even when the ignition is off. This memory is powered by the battery at all times. When the battery is disconnected, KAM is erased, and sometimes the ECM sets P0603 as it relearns. In other cases, the ECM's internal KAM chip has actually failed.

Affected Models

All vehicles 1996+
Common in Ford and Lincoln vehicles
Common in GM vehicles
Common in Chrysler and Dodge vehicles
Often appears after battery replacement or disconnection

Common Causes

Battery disconnected or dead for an extended period, erasing KAM data
Weak or failing battery causing repeated KAM data loss when voltage drops
Internal ECM failure causing the KAM memory chip to fail permanently
Parasitic battery drain repeatedly killing battery and erasing KAM
Faulty ECM power supply relay causing intermittent power loss to the ECM

How to Fix It

If the code appeared after battery replacement or disconnection, it may clear on its own after several drive cycles. Drive the car normally and see if the code goes away.

After battery disconnection, the ECM needs time to relearn fuel trims and idle values. This can take 50 to 100 miles of varied driving.
Check battery health and connections. A battery that drops too low overnight will erase KAM repeatedly. Test the battery and replace it if weak.

A battery dropping below 10 volts while parked can erase KAM and set P0603.
Check for a parasitic electrical drain if the battery keeps going dead. With the car off, disconnect the battery negative cable and measure current flow with a multimeter in series.

Normal parasitic draw is under 50 milliamps. Higher draw indicates something is staying on and draining the battery.
Inspect the ECM power supply fuse and relay. A relay that intermittently loses power to the ECM will cause repeated KAM erasure and this code.

The ECM has two types of power feeds — ignition-switched power and constant battery power for KAM. The constant feed must be stable.
If the code persists through multiple drive cycles with a healthy battery and no parasitic drain, the ECM itself has an internal memory failure and needs replacement.

Have a shop confirm the diagnosis before replacing the ECM — it is an expensive part.

When to Call a Professional

If the code returns after battery reconnection and a few drive cycles, the ECM may need replacement. A remanufactured ECM costs $150 to $500 depending on the vehicle. ECM programming adds $100 to $300 at a dealer. If a parasitic drain is draining the battery, diagnosis costs $75 to $150 at a shop. Always fix a parasitic drain before replacing the ECM to avoid a repeat failure.

Frequently Asked Questions

Is P0603 serious?

It depends on why it set. If it is just from a battery disconnection, it is not serious and will likely go away on its own. If the ECM's memory has truly failed, it is serious — the engine may not perform correctly long-term. Start with battery checks before worrying about ECM failure.

What does 'Keep Alive Memory' actually do?

The ECM constantly learns how to fine-tune the engine for your specific car. Things like how much fuel to add, how high to set the idle, and when to shift are stored as learned values. KAM keeps these values when the car is off. If KAM is erased, the engine relearns from scratch — this can cause rough idle or poor fuel economy briefly.

Will clearing the code fix P0603?

Clearing the code may help if battery issues were the cause. But if the ECM's memory chip has failed, the code will return immediately or after the next drive cycle. Fix the root cause first — do not just clear and ignore.