P0603 is a generic OBD2 trouble code. It indicates that your Nissan Altima’s keep alive memory has had an error.
Keep alive memory is the data that your Altima’s PCM (Powertrain Control Module) has learned about how your engine conditions. All of this sensor and driving input data is used to keep it operating efficiently. Keep alive memory needs power from the battery to keep the data stored, and will delete with power from the battery.
When the keep alive memory fails (such as when you unhook the battery) your Altima falls back onto the default values that were hard programmed into the memory from the factory. Typically, there won’t be a noticeable difference in engine performance when this happens unless the PCM is compensating for an engine issue.
P0603 Symptoms: Nissan Altima
The only symptom of P0603 may be the check engine light. This is particularly true if the keep alive power circuit has failed. You may notice that engine stalling or your Altima may not start at all.
P0603 Causes + Diagnosis: Nissan Altima
Here are the most common causes of P0603:
Battery
A great place to start seeing what is causing P0603 in your Altima is the battery terminals. If they are loose, damaged, or corroded it can reset the keep alive memory. This will force the PCM to fall back onto the factory specs and throw the code.
Take a look at the battery cables and make sure they are free of corrosion and tight. If they are, make sure that the wiring from the battery terminal to the fuse box looks good. Make sure that the ground wiring isn’t corroded.
Charging System
A good trick to see if the charging system is at fault is to clear the code (you’ll need a good scanner) and disconnect the alternator. Now start your Altima and see if P0603 comes back. If it does that indicates that you probably have a bad alternator or voltage regulator.
Ignition System
Check the wiring harness around the coil packs. Look at the plug wires. Check the coil packs themselves for cracks. You are looking for something that might be throwing voltage onto your Altima’s wiring harness.
A tried and true trick is to wait until it’s dark and start the engine. You should see the spark “bleed” from the ignition system if there is a problem with it.
Bad PCM
A bad PCM can cause P0603, and often does. Water damage is a particularly common culprit. Vibration or a power surge can also harm it. Don’t start with the PCM, but if you can’t find any noticeable issue, it very well could be the problem.
KAPWR Circuit
The keep alive memory circuit may have an open or short circuit. If that’s the case the keep alive data won’t be able to record at all. Check the voltage and wiggle the harness. See if it stays consistent.
Conclusion: P0603 Nissan Altima
P0603 can be a tricky code to track down. It’s going to be a wiring issue or a bad PCM. If there is anything that you would like to add that could help someone with their Nissan Altima, please leave a comment below. Good luck!