2004 hyundai elantra sedan check engine light on, 1st try at autozone gave P2251 followed by P0420, I reset the car computer to check if it comes back and it did after nearly 100 miles. second try at autozone gave p0420 but the guy looking at it is was not confident at all. in both cases the pcode did not appear directly on the obd scanner. he connected it to a pc to see the codes. There is no misfire, lack of power, same mileage, no smoke. I'm not sure y the code is coming.

What code did autozone say was in the PCM ? Did it have the word circuit in the discripition ? Maybe wiring issue or PCM etc...

You got ripped off by the mechanic. A good mechanic can pinpoint the problem by using a scan tool. Take vehicle to autozone or similar and have it scanned for codes this will give you a idea what is wrong. I believe the problem is exhaust related because of the blinking light.

The information below was complied by me, it explains what causes the check engine light to come on and the process of finding the cause.

When the Check Engine light (or Service Engine Soon) is turned on, the systems computer control unit (called the PCM) has sensed a fault in the electronic controls of the engine and/or the emission system. When this happens you will seldom feel the car engine run any different. There are about 200 possible system faults (the gas cap is just one of them) that turn the Check Engine light on, to know what has developed a problem you must have the system tested, this is called an OBD2 fault code test, Kragen/Oriley and Autozone parts stores (USA only) do this testing for free.

P2251 L.O2 reference circuit(-) open-Sensor 1. This code is showing a problem on the reference voltage of the upper O2 sensor. It might be sensor or the wiring. P0304 Cyl.number 4 Misfire detected. This code is showing 4th.cylinder is or has been missing. Second code might be set by first code.O2 sensor problem needs to be corrected first.

you can diconnect the positive battery cable for 30 secs. or take it to autozone they can reset it for free. try waiting a day or two the engine must go through so many cycles without any problems before the light will go out. If it doesnt go out there is still a problem. More than likely if you changed catalitic converters and the light stays on it probaly has a bad O2 sensor.

Hi, for the P0140 see http://www.obd-codes.com/p0140. For the P2251 see http://www.elantraxd.com/forums/showthread.php?20258-P2251-O2-sensor

http://www.aa1car.com/library/p0420_dtc.htm

Did the 'Check Engine' Light come ON? Whenever the 'CHECK ENGINE' light comes on, the ECU (Engine Control Unit) sets a Diagnostic Trouble Code (DTC). These codes identify the particular engine management system which caused the light to come on.
You'll need to use/find an OBDII (On-Board Diagnostic) Scanner to retrieve any code which have been stored in the ECU computer. Afterward, we might be able to help you DIAGNOSE the problem and perhaps eventually fix the problem.

Danny,

Catalytic Converter Failure is a common cause of a P0420 Code. 1.) I would give a few calls to some muffler shops and get a thorough exhaust inspection done. As a leaking/damaged Exhaust Manifold, Catalytic Converter, or exhaust pipe could be some of the culprits as well. If it were any of those other items your car would give you a different OBD Code. Item #2.) This is not common with a P0420 Code. With the P0420 code you should have no driveability issues. (not to say that you wouldnt in an extreme rarety) Item #3.) Again this would flag an entirely different code such as P0455 (Large Evap Emission Leak) Item#4.) Engine Oil would not even set a code. I hope this helps you out, and thank you for using FixYa!

Sincerely,

JC

I have a 2004 Hyundai sonata I hear a squealing sound and the car looses power when i start to go. The error code is P0301 any help would be great.
Thanks