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The list of codes from cycling the ignition key On - Off - On - Off - On within 5 seconds does not show all the corresponding OBDII codes. The list shown below has been compiled from the FSM, the Powertrain Diagnostic Manual, and online sources.
Since the 1995 Neon is not fully OBDII compliant, that list is shown separately. Some 1995 key dance codes have no corresponding OBDII codes. I have a 1995 Diagnostic Procedures Manual which also covers other body styles with 2.0L and 2.4L engines so it is possible there may be small errors on that list. Send a PM if you have any corrections, especially if you scan an OBDII code that isn't listed.
Using OBDII scanner codes is more specific when there are several OBDII codes for a key dance code.
The OBDII connector is right under the lower left edge of the drivers side dash. You can purchase an OBDII scanner or you can get a free scan at many parts places. They do this service to generate parts sales but don't feel compelled to throw parts at codes because many of them are caused by wiring problems. See the sticky about codes for fixes and if you can find a factory Powertrain Diagnostics Manual, you can narrow your problem down even more. The 1995 Powertrain Diagnostics Procedures Manual is as thick as an FSM and has more illustrations and charts which would be useful for any year Neon. The purpose of this post is to help eliminate guessing and possibly save time and money.


1995

11
No Crankshaft Reference Signal At PCM
P1390 Timing Belt Skipped Tooth
P1391 Intermittent Loss Of CMP Or CKP

12
Battery Disconnected

13
P1297 No Change In MAP From Start To Run

14
P0107 MAP Sensor Voltage Low
P0108 MAP Sensor Voltage High

15
P0500 No Vehicle Speed Sensor Signal

16
Knock Sensor Circuit

17
P0125 Closed Loop Temperature Not Reached

21
P0131 Upstream O2 Sensor Voltage Shorted To Ground
P0132 Upstream O2 Sensor Shorted To Voltage
P0133 Upstream O2 Sensor Slow Response
P0134 Upstream O2 Sensor Stays At Center
P0135 Upstream O2 Sensor Heater Failure
P0137 Downstream O2 Sensor Voltage Shorted To Ground
P0138 Downstream O2 Sensor Shorted To Voltage
P0139 Downstream O2 Sensor Slow Response
P0140 Downstream O2 Sensor Stays On At Center
P0141 Downstream O2 Sensor Heater Failure

22
P0117 ECT Sensor Voltage Low
P0118 ECT Sensor Voltage High

23
P0112 IAT Sensor Low
P0113 IAT Sensor High

24
P0121 TP Sensor Voltage Does Not Agree With MAP
P0122 TP Sensor Voltage Low
P0123 TP Sensor Voltage High

25
P0505 IAC Motor Circuits

27
P0201 Injector No. 1 Control Circuit
P0202 Injector No. 2 Control Circuit
P0203 Injector No. 3 Control Circuit
P0204 Injector No. 4 Control Circuit

31
P0443 EVAP Solenoid Circuit
P0441 EVAP Purge Flow Monitor Failure

32
P0401 EGR System Failure
P0403 EGR Solenoid Circuit

33
A/C Clutch Relay Circuit

34
Speed Control Solenoid Circuit

35
P1491 Fan Control Relay Circuit

37
P1899 P/N Switch Failure
P0740 Torque Converter Clutch, No RPM Drop At Lockup
P0743 Torque Converter Clutch Solenoid Circuit

41
Generator Field Not Switching Properly

42
Fuel Pump Relay Control Circuit

43
P0300 Multiple Cylinder Misfire
P0301 Misfire Cylinder No. 1
P0302 Misfire Cylinder No. 2
P0303 Misfire Cylinder No. 3
P0304 Misfire Cylinder No. 4
P0351 Ignition Coil No. 1 Primary Circuit
P0352 Ignition Coil No. 2 Primary Circuit

44
Battery Temperature Sensor Volts Out Of Limit

46
Charging System Voltage Too High

47
Charging System Voltage Too Low

51
P0171 Fuel System Lean

52
P0172 Fuel System Rich

53
P0601 Internal Controller Failure

54
P0340 No Cam Signal At PCM

55
DTC Fault Display Complete

62
PCM Failure SRI Mile Not Stored

63
PCM Failure EEPROM Write Denied

64
P0420 Catalytic Converter Efficiency Failure

65
P0551 Power Steering Switch Failure
P0703 Brake Switch Sense Circuit

71
P1496 5 Volt Supply Output Too Low

77
Speed Control Power Circuit



1996-99

11
P0320 No Crankshaft Reference Signal At PCM
P1390 Timing Belt Skipped Tooth
P1391 Intermittent Loss Of CMP Or CKP
P1398 Misfire Adaptive Numerator At Limit

12
Battery Disconnected

13
P1297 No Change In MAP From Start To Run

14
P0107 MAP Sensor Voltage Too Low
P0108 MAP Sensor Voltage Too High
P1496 5 Volt Supply Output Too Low

15
P0500 No VSS Signal

16
P0325 Knock Sensor Circuit

17
P1281 Engine Cold Too Long
P0125 Closed Loop Temperature Not Reached

21
P0131 Upstream O2S Shorted To Ground
P0132 Upstream O2S Shorted To Voltage
P0133 Upstream O2S Slow Response
P0134 Upstream O2S Stays At Center
P0135 Upstream O2S Heater Failure
P0137 Downstream O2S Shorted To Ground
P0138 Downstream O2S Shorted To Voltage
P0139 Downstream O2 Sensor Slow Response
P0140 Downstream O2S Stays At Center
P0141 Downstream O2S Heater Failure

22
P0117 ECT Sensor Voltage Too Low
P0118 ECT Sensor Voltage Too High

23
P0112 IAT Sensor Voltage Low
P0113 IAT Sensor Voltage High

24
P0121 TP Sensor Signal Does Not Agree With MAP
P0122 TP Sensor Voltage Low
P0123 TP Sensor Voltage High

25
P0505 IAC Motor Circuits
P1294 Target Idle Not Reached (+/-200 RPM)
P1299 Vacuum Leak Found (IAC Fully Seated)

27
P0201 Injector Circuit No. 1
P0202 Injector Circuit No. 2
P0203 Injector Circuit No. 3
P0204 Injector Circuit No. 4

31
P0441 EVAP Purge Flow Monitor Failure
P0442 EVAP Monitor Small Leak Detected
P0443 EVAP Purge Solenoid Circuit
P0455 EVAP Monitor Large Leak
P1486 EVAP Monitor Pinched Hose Found
P1494 Leak Detection Pump Switch Or Mechanical Fault
P1495 Leak Detection Pump Solenoid Circuit

32
P0401 EGR System Failure
P0403 EGR Solenoid Circuit

33
P0645 A/C Clutch Relay Circuit

34
P1595 Speed Control Solenoid Circuit
P1596 Speed Control Switch Always High
P1597 Speed Control Switch Always Low

35
P1491 Radiator Fan Control Relay Circuit

37
P0740 TCC No RPM Drop At Lockup
P0743 TCC Solenoid Circuits
P1899 P/N Switch Stuck In Park or Gear

41
P0622 Generator Field Improper Switching

42
P1282 Fuel Pump Relay Control Circuit
P1388 ASD Relay Control Circuit
P1389 No ASD Relay Output Voltage At PCM
P0460 Fuel Level Sending Unit & Circuit, No Change Over Miles
P0462 Fuel Level Sending Unit Voltage Out Of Limits, Too Low
P0463 Fuel Level Sending Unit Voltage Out Of Limits, Too High

43
P0300 Multiple Cylinder Misfire
P0301 Misfire Cylinder No. 1
P0302 Misfire Cylinder No. 2
P0303 Misfire Cylinder No. 3
P0304 Misfire Cylinder No. 4
P0351 Ignition Coil No. 1 Primary Circuit
P0352 Ignition Coil No. 2 Primary Circuit

44
P1492 Ambient/Battery Temperature Sensor Voltage High
P1493 Ambient/Battery Temperature Sensor Voltage Low

46
P1594 Charging System Voltage Too High

47
P1682 Charging System Voltage Too Low

51
P0171 Fuel System Lean

52
P0172 Fuel System Rich

53
P0600 PCM Failure SPI Communications
P0601 Internal Controller Failure

54
P0340 No Cam Signal At PCM

55
DTC Fault Display Complete

61
P0106 Barometric Pressure Out Of Range

62
P1697 PCM Failure SRI Mile Not Stored

63
P1696 PCM Failure EEPROM Write Denied

65
P0551 PS Pressure Switch Failure
P0703 Brake Switch Stuck, Pressed, Or Released

71
P1496 5 Volts Output From Regulator Low

72
P0420 Catalyst Efficiency Below Required Level

77
P1683 Speed Control Power Relay Circuit

Printable .doc files for your glovebox:
'95 codes
'96-'99 codes

How is it even possible to get a Code 12, battery disconnected code?? How would the CEL light even flash??

It's that the battery has been disconnected, not is disconnected. LOL! And it won't light the CEL if it is set. It goes away after 50? keyswitch cycles. It's just there to make it clear that all codes might not be showing, as the PCM has been cleared fairly recently.

Code 12 will appear the next time the battery is connected, not while the battery is disconnected. It will only show up on the key dance, not as a CEL (check engine light). Code 12 will reset itself after about 50 normal on off ignition cycles with the engine reaching operating temps each time. An intermittant short will also cause a Code 12.
Some scanners will show it as a "not ready" meaning that resetting the PCM will not allow you to try to pass an inspection right after resetting the PCM, but rather until the PCM has relearned all of its settings which can take a few days and if there is a malfunction, it may show up again by the time the PCM reaches the "ready" state.
Edit:
Code 12 will also show up anytime there are other codes. When it precedes other codes, it just indicates that there are stored codes, not necessarily that the battery has been disconnected. Anytime the CEL comes on, a code 12 will just indicate the start of stored codes and then code 55 indicates the end of stored codes.

Flashes but doesn't set a code... can't remember what that was, just that it was bad... Save me Case, I have somebody PMing me... was it just mis-fire that does that?

If the check engine light is flashing, that means you should shut it down. Catalytic converter damage may occur if too much raw gas from a misfire overloads the converter. Everytime I have seen a CEL flashing, it was a misfire.
Some people just have a difficult time to get the key dance to work. That doesn't mean a code isn't set. An OBDII scanner should be used if anyone who can't get the keydance to work. On my '03 Dakota, I can get the key dance to work about every third time I try, but no problem getting the key dance on any of my Neons. Over the years, there have been a few posts from people not being able to get a key dance code.

Thanks Case, I thought it was a misfire. Just wanted to make sure. The guy's in my neighborhood, so I'll see if I can't get him to come by this weekend, and straighten it out. Helps having spare parts to figure out the difficult ones.

Again, you're a life saver!

Feel free to use the link at the end and in my sig for the printable .doc file. It fits very nicely on 3 pages and is real nice to have in your glove box. I have a copy in all 4 of my Neons.

When a scanner reports "not ready", there are specific tests that the PCM must finish before it can say "ready". This can be a problem if you had disconnected your battery or reset the PCM and need to have your Neon tested for emissions. Your driving style may have an effect on how long the process takes to achieve "ready". The following link describes exactly what it takes for the PCM tests:
Chrysler drive cycle
My son had this problem for over a month and followed the conditions for the tests exactly and was able to achieve the "ready" status to pass emissions testing in Milwaukee.

thanks for making this info readily availible to us. it sure helped me figure out the missfire problem in my 96!! i had no idea that it could mess up the cat. converter. and thats a great idea about keeping a code list in your neon. im gonna do just that now! oh and hey, my girl has a caravan, a 93, are the "dance" codes the same on it? cuz i did the dance on it when running like crap and the code shot back was one for the O2 sensor. i replaced it and it runs great now. just wondering.

A '93 Caravan will not have OBDII codes although it will have the same key dance method for helping diagnose a CEL. The key dance codes may have different meanings so it would be best to search for the appropriate FSM (factory service manual). OBDII was required on all '96 and newer vehicles.