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Trouble code P0420 means: Catalyst system efficiency below threshold (bank 1)
warning code check faults indicated by subsequent codes
Trouble code P0171 means:System too lean (Bank 1)
Trouble code P0301 means: Cylinder 1 Misfire Detected
It means you need to go to Autozone or Shucks/Kraagen and have them read the codes with the diagnostic code reader which they will usually do free of charge. Something in or on the engine is malfunctioning and needs repair/replacement. The diagnostic code will tell you where to look for the problem. Oxygen sensor is an example of typical problem discovered but all engine sensors are monitored.
This car is an "OBD2" model, and requires a code reader or scan tool to access the information in any of the car's computers. Many auto parts stores will do this for you for free...just don't take their word for what's wrong.
In the glove box.
The 1996 Mystique has OBDII (On Board Diagnostics Level 2) and requires a command from a "code reader" or OBDII monitor. It can also be shut off by disconnecting the battery for a long enough period of time, apparently - but I have an Auto XRay unit, so I just use that. If the cause is not repaired, the CEL WILL come back on.
You need a OBD2 scanner/code reader to get stored codes and to erase them
I have a 1998 and the code is in the back where your jack and accessories are.
passenger side panel, above left knee
With an OBD2 code reader