Code PO303 is listed as a number three spark plug misfire and PO421 lists as a catalytic converter problem.
The trouble with codes is it isn't often the code lists the actual fault and some further tests and detective work are needed. Sometimes the ECU is actually telling lies about the code it has logged.
I suggest best practice which is basics first. Make certain the service work is up to date and the spark plugs are in good order with an adequate spark, filters clean and so forth and finish with a thorough visual inspection to ensure everything is as it should be then clear the fault codes and go for a drive.
If the codes are repeated further checks with a more sophisticated scan tool, oscilloscope and dedicated sensor testers should be carried out. At this stage it is often cheaper in the long run to put the car in the hands of an expert diagnostician than to struggle along alone.
On that car the P0303 indicates a misfire on cylinder 3 either from a fouling / flooded plug or from a break in a spark plug wire. the other code is for your oxygen sensor you may want to run a can of Seafoam through the thing first and see if that stuff blowing all the varnish and deposits off of the plugs you may get away without having to replace those things right now if you do.
SOURCE: I have problem on airbag codes in mitsubishi lancer
Your first code is for low system voltage. If your battery has needed a jumpstart lately or replacement, this code will be set.
The second two codes are in relation to your rh side airbag (B1C2D) and curtain bag (B1B22) not being visible to the srs ecu. Has the car been apart for any reason, possible leaving harnesses unplugged? Was it in an accident recently and maybe the bags are missing? Were components removed for accessory installations (stereo, subwoofers, etc)?
The connector for the curtain bag is in the passenger side rear most pillar. The connector for the side airbag is under the passenger seat.
A weak battery can sometimes throw other miscellaneous codes like these, but it is not likely. If you clear the codes, the B1C2D and B1B22 should come right back.
If they do, and both bags are installed correctly, You can bypass them with a 3 ohm (three ohm) resistor and clear the code again. If the codes set again, there is a problem with the wiring leading up to the squib units. If the codes do not set again, there is a problem with the squib units.
SOURCE: lancer not getting enough fuel and cutting out
It might be the fuel filter needs to be changed and it's cutting off flow. Other wise you'll have to look at the pump or injectors.
SOURCE: 2002 mitsubishi lancer oz rally troubleshooting
Code p0420 is 'catalyst efficiency below threshold'.
This could be caused by a bad catalytic converter but before dumping a ton of money on that I would suspect the after-cat oxygen sensor is getting lazy and setting the code.
Have the O2 sensors checked and replace if needed before suspecting the catalytic converter.
Good luck.
SOURCE: getting p0303 code. replaced the plugs, problems
Not sure how many miles on your PT,but may be wire bad or not fully seated or bad lug on coil for #3 cylinder.Not sure if crankshaft position sensor would cause only one cylinder to missfire.Just remember that the timming belt needs to be replaced between 75,000 and 100,000 miles.My wife didn't know this.Well hope this helps,and good luck.
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These readings are very accurate..
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