1997 Dodge neon radiator fan

check fan relay in fuse box. have it tested at auto zone .Check that no wires are loose going into the fan.

Here's a link to the P0340 code,and P1491.Powertrain Diagnostics Manual P-1491 Radiator Fan control relay circuit
Set condition: An open or shorted condition is detected in the radiator fan relay control circuit.

Possibilities:
Fan relay ground circuit open
Fused B+ circuit open
Rad fan control relay defective
RF relay output circuit volt cycles over to batt voltage
RF relay control circuit open
RF relay ctrl rly ckt wir harness observable or intermittant defect
RF relay ctrl ckt short to ground
Defective PCM

Get out your Multimeter.

It is common for some relay positions to not be used on many cars. These are places for relays that control things that your vehicle is not equipped with. For example, if your vehicle is not equipped with fog lamps, the spot where the fog lamp relay would go will be empty and there will be no connectors in that spot either.

As for your overheating problem, it needs to be properly diagnosed. Most Dodge engines are VERY unforgiving when it comes to being overheated. Even if it was not caused by a blown head gasket, there is a really good chance that it has one now.

Combustion gasses in your cooling system can keep your fans from running, because the gasses push the water down and away from the Engine Coolant Temperature (ECT) sensor that tells the Powertrain Control Module (PCM) when to operate the fans.

Your Avenger is actually equipped with four different fan control relays. It is very unlikely that all of the relays would be bad (or missing).

Proper diagnosis required.

Does the cooling fan come on ? Not the thermostat . Sitting still no air movement through the radiator ,when you drive air flows through the radiator. I would be looking at a problem with the cooling fan . Relay maybe, fan motor etc...

So the car only overheats at idle? How many miles on the car? What fan are you referring to - the heater fan or the cooling fan for the radiator? It sounds like you may have a plugged up radiator. If the engine has high mileage the radiator may have accumulated enough junk over the years to become restricted. Some automotive technicians have a laser that they can use to check the radiator for cool spots, which would indicate internal blockage.

if fans not woking when AC is turned on check the multiplug connections on the fans first they corrode and burn the contacts inside the plug

Either the fan is out or the (Relay/Fuse)

You might also have a problem with the connecting cable to supply the fan with power, check with a proper voltmeter.

Hope that helps.

What is the mileage? Check the reservoir for coolant, when the engine is COLD,... I repeat COLD, check the radiator for fluid. If fluid is present, check to see if your fans are working properly, turn your AC on and your fans in front of your motor should kick in within minutes.

If you have coolant in the reservoir and radiator (suspect a new thermostat, replacing the radiator will benefit greatly too)

If your fans do not turn on within minutes, replace your radiator fan relay.

1997 Plymouth Neon, have replaced radiator, radiation cap, thermostate and hoses, still run hot. Fans do not seem to come on, where is the temperature sensor on this vehicle?

First, the easy way is to just bypass the relays all together. Install a switch on the inside of the car like you would fog lights. This will solve the problem of replacing thing after thing and wasting money. Just a $7.00 fix. After doing that unplug all the sensors and then the battery for at least 60 second to reset the computer. Than restart the car and the check engine light should go out, may take a day or so. Same thing with your gas cap, re-securing it will take a day or so for the light to go out. Good luck. I love my '97 neon.