Having problems with your 2003 Pontiac Grand Am ?

Showing PO420 and PO304 code

\015 I have a 03 grand am with 3.1 and it is showing PO304 and 420 code. I know that the 420 code is a code thats bad about showing on the chevy engine b/c i sale exhaust parts. Also some times when taking off, like when getting on to the interstate it bogs down and you have to let off the gas and then get back on it for it to stop doing that. Any ideas? someone told me saids like a MAP sensor.\015 Posted / edited by AnonymousUser on : 02-12-2019

Answers :

MAP sensor, Maybe, but doubt it.
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\015\012When the misfire monitor detects a misfire, it will trigger the check engine light with the specific cylinder number as the last digit in the P030X code. For instance cylinder 1 misfire is P0301, cylinder 2 is P0302 etc. In this case we are left with a P0304 in the computer memory. Which is cylinder 4.
\015\012Possible Causes:
\015\012
\015\012Fuel injectors, related wiring, sensors and computer issues
Running out of gas, or poor fuel quality
Evaporative emissions system (EVAP) concerns: fuel vapors leaking into engine
Incorrect Fuel Pressure
EGR system concerns: leaking EGR valve or restricted ports
Base engine concerns: low compression, valve train problems and timing issues
Ignition system concerns including, but not limited to:
Faulty spark plugs
Faulty coil or related wiring
Ignition module or related wiring issues
Ignition related sensor faults or wiring issues
\015\012This maybe causing PO420 code to go off.
\015\012Which is:
\015\012P0420 Catalyst System Efficiency Below Threshold (Bank 1)
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\015\012There is one O2 (oxygen) sensor in front of the catalytic converter (called upstream), and one behind it (downstream). When the engine is warmed up and running normally the upstream oxygen sensor reading fluctuates and the downstream one is fairly steady. In the case where both readings are fairly steady, a P0420 check engine light code is set.
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\015\012Possible Causes:
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Oxygen (O2) sensor not working correctly
Damaged or leaking exhuast (pipe, manifold, catalytic converter)
Damaged catalytic converter
0304 is a missfire on #4. that could be causing the other code. replace the #4 plug (or all) and also replace left 02 sensor. disconnect battery to clear codes. good luck
Repair Help & Product Troubleshooting for 2003 Pontiac Grand Am

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