What P0603 means
Keep alive memory holds learned values – fuel trim, idle and shift adaptation, readiness status – while the engine is off, as long as the module has constant battery power; CARB defines it as memory that is lost when the battery or fuse is disconnected.
How the engine computer detects it
The module stores a checksum of this memory at shutdown and compares it at the next start (Ford, GM Europe); GM calls the check 'long term memory reset', and the Ford diesel watches the supply of its engine-off timer chip (below 0.25 V). By far the most common trigger is outside the module: a disconnected, flat or replaced battery or an interrupted constant feed – Nissan bulletins name exactly this and ask for at least 11 V before clearing. Learned values then start from default and the readiness monitors are reset. Whether the lamp comes on depends on the maker.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light on (depending on the maker)
- Rough idle or changed shifting until values are relearned
- Readiness monitors reset
- Often right after battery work
Common causes
- Battery disconnected, deeply discharged or replaced Most likely
- Interrupted constant power feed to the module (fuse, wire, corrosion) Possible
- Main relay drops out too early, memory not saved completely at shutdown Less likely
- Internal memory defect (EEPROM/NVM) Less likely
How to diagnose P0603
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: engine control module, battery, control module power supply.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 6
- Computer and output circuits
- 03
- Specific fault
Questions drivers ask
Can I drive with code P0603?
Usually yes for a short time, if the car drives normally, the temperature gauge reads normal and the check engine light is steady. If the light flashes, the engine loses power or overheats, or other warning lights come on, pull over and have it checked.
Will my car pass an emissions inspection with P0603?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
Can I fix P0603 myself?
Often yes, with an OBD-II scanner and basic tools. Difficulty: easy. Start with the cheapest checks before replacing expensive parts.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.