Has code 0420 - 2002 Dodge Neon

code p 0300 refers to random /multiple cylinder/s ---misfire detected ===causes spark plug/s---HT lead/s ---injector/s --ignition coil/s ---low compression---wiring
code p 0316 refers to misfire detected during start up--first 1000 revolutions ===causes --engine mechanical fault ---wiring---ignition system---fuel system--injector/s
code p 0420 refers to cat converter system, bank 1---efficiency below threshold === causes cat converter---HO2S 2
code p 430 refers to the same problem and causes as code p 0420
Here is a news flash .
engines under the control of an engine control module (ECM) do not need you to do a tune up as it tunes the engine 60 times a minute and reports to you by fault codes when it can no longer tune up the engine

Here's your trouble code.

There are numerous things that can cause this code to come up.First how many miles do u have? U have to make sure there are no exhaust leaks by the down stream O2 sensor. Also have to make sure there are no fuel delivery problems or vacum leaks. If u have access to a scanner for your vehicle u will have to look at your fuel trims and make sure they are around 0% at idle and off idle. Also have to make sure your O2 sensors are working correctly. Post some more info for your vehicle and i can guide u in the right direction

A real smorgasbord!
0420 is for the cat, so scrub that one. I could not find 0517-don't know what that means. 0551 and 0128 are sensor malfunctions, or circuit malfunctions for those two sensors (P/S pressure switch, and Coolant temp. sensor). Before going after that, take heed of 0622 and 0563: 0563 is system voltage too high, 0622 is generator field "F" control circuit malfunction. You have a bad voltage regulator in your alternator-putting out too much current. Try a different alternator-it may clear up the sensor malfunctrions, too.

Trouble code P0420 means: Catalyst System Efficiency Below Threshold

This is unrelated to the noise when turning sharply or the grinding when braking/brake light.

Follow this link, scroll down the page to see the list of codes. Good Luck.
http://www.trouble-codes.com/

if they are PO codes the engine is close to fail the 0510 is a closed throttle when other sensors think it should be open

Code P0420 is a catalist ineficiency code. It can be caused by a minor exhaust leak. Based on what youve already replaced. Inspect the entire system while its running to find any leaks.

Here is something from a reference guide that we follow at my shop, hope it helps.

When diagnosing a PCM code PO420, be sure to determine the cause of the converter failure. Converters are not likely to fail without an external cause. Typical causes are: fuel pressure regulator leak, engine misfire, dirty fuel injectors, coolant consumption, etc. Also, be aware that some converters on the market will cause this code or codes due to a lack of oxygen storage capacity. A component that must be in these converters, Cerium, is required to store, then slowly release oxygen.

Hello engelhard58
I have attached a link that can give you some of the reasons why your getting that code.I hope you able to get your answer out of that.
Mean of the code=P0420 Catalyst System Efficiency Below Threshold.
Link to site