1997 Dodge Neon P0505 code

Were you able to restart the engine? Will it stay running if you hold the accelerator pedal down 1/4"? Has the battery recently been disconnected or run dead.

The most common cause of code 505 is someone disconnected the idle speed motor while the ignition switch was turned on. The second most common cause is spread terminals in the connector that make intermittent contact. You also have to consider a wire rubbed through and grounding out. The idle speed motor itself has a very low failure rate. Almost all failures are due to internal mechanical problems that aren't monitored by the Engine Computer and won't set a fault code. Electrical problems inside the unit that set a fault code are very rare.

The troubleshooting steps you're looking for are rarely used by mechanics because they are way too time-consuming. They might be okay for people who don't understand electrical, but there are way too many instances where those steps walk you right over the cause of the problem. Typically they will tell you to use an ohm meter to test each of the four wires between the Engine Computer and the idle speed motor. Instead, if you want to go through all that, just unplug the connectors from the computer and measure between each of those four wires and at least any one of the other three wires. If any test shows an open circuit, then you can dig deeper to see which wire has the break.

Dandy. Unless there's some other problem, this happens on all Chrysler products and the fix is real difficult.

The Engine Computer loses its memory when it loses power, as in disconnecting the battery. It has to relearn "minimum throttle" before it will know when it has to be in control of idle speed. Until that happens, it is usually hard to start unless you hold the gas pedal down about 1/4", it will tend to stall at stop signs, and it will typically stall when you put it in gear or turn on the air conditioning.

Now for that fix. To meet the conditions for the relearn to take place, drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the pedals.

Then there's one of two things. The brake light switch could be slightly out of adjustment or there's a vacuum leak. Try the coasting procedure while holding the brake pedal up with your foot.

To verify a vacuum leak, you can use a scanner to view live data and look at the "step" the Engine Computer has placed the idle speed motor at. For a good-running engine, step 32 is typical. If you find it on step 0, the relearn hasn't taken place. If you find it on a lower step, such as step 10 to 20, for example, the computer has commanded the idle speed motor to close its controlled / designed-in vacuum leak because air is sneaking in somewhere else that it has no control over, meaning a leaking vacuum hose, gasket, or part that operates on engine vacuum.

If a vacuum leak is suspected, you can pinch off various hoses to see if doing so brings the idle speed down. You can listen for the hiss of a leak, or you can spray water over the engine to see if it gets sucked in somewhere.

Erratic idle speed can also be caused by the air passage the idle speed motor controls getting plugged with carbon, but that is very uncommon now with the better additives in today's gas. You need to unbolt and remove the idle speed motor to look in the passage to see if it's clear. You'll need a high-quality Torx bit for the two screws because they have thread lock sealant on them and they turn out very hard. You don't want to round out those heads.