P0603 Code Meaning and Next Steps

First checks take 10 to 20 minutes for the first checks. No special tools are usually needed for the first checks.

Can you keep driving?

Stop driving if any of these apply

!The engine starts stalling, losing power sharply, or the warning light flashes.
!The vehicle begins to run erratically or enters limp mode while the code is active.

If the light is steady and the vehicle still drives normally: Often yes for a short time, but it should not be ignored.

What to check first

Step-by-step checks

1
Free - no tools
Check battery condition and charging system history before replacing the module
2
Free - no tools
Inspect module power and constant-feed connections for looseness or corrosion
3
Basic tool needed
Notice whether the code appeared after a dead battery, jump start, or battery disconnect
4
Basic tool needed
If the vehicle has lost other learned values or settings, that supports a power-loss or memory issue
5
Basic tool needed
If scan data is available, capture the code before clearing it so you know whether it is repeatable

If the code returns

-If the battery or supply side is weak, fix that before condemning the module.
-If the code returns immediately after being cleared, internal module memory trouble becomes more likely.
-If the module still loses memory after power is stable, replacement may be required.

Background

What this code means

P0603 is a generic OBD-II code for control module keep-alive memory error.

The module uses keep-alive memory to remember settings and learned values when the key is off, so voltage loss or internal memory failure can trigger the code.

Diagnosis

Common causes

Most common

Battery or power interruption

Voltage loss can wipe or corrupt keep-alive memory.

Common

Module constant-feed issue

A poor always-hot supply can prevent memory retention.

Common

Ground problem

Weak ground can cause memory or logic faults.

Possible

Internal module memory failure

The module may no longer retain its stored data correctly.

Avoid these mistakes

What not to do

xDo not replace a control module first if there is obvious wiring, connector, or power-supply damage.
xDo not assume a module code always means the module itself is bad before checking the supporting circuit.

Parts

Parts that may need replacing

PartTypical costNotes

Battery or charging system test$0-$50Often the first check before module replacement.

Constant-feed wiring repair$20-$150Relevant when the memory supply is weak or intermittent.

Control moduleVariesOnly after power and ground checks are confirmed.

Related OBD codes

P0601

P0601 usually means the internal control module memory checksum is incorrect.

P0602

P0602 usually means the control module has been programmed incorrectly.

P0604

P0604 usually means the internal control module random access memory has failed.

Source notes

Generic OBD-II (SAE J1979 / ISO 15031-5). P0603 was expanded around keep-alive memory faults, especially battery events, constant-feed issues, and internal module memory failure.

This guide is written as a generic multi-make reference, so bulletin history, sensor locations, and repair order can still change by manufacturer and engine family.

This is generic OBD-II guidance and should not override vehicle-specific service information. Exact diagnosis and repair steps vary by make, engine family, and model year.

Last reviewed: 2026-04-10

Reference: Open reference

P0603 - P0603 Usually Means the Control Module Keep-alive Memory Has Failed