Chrysler Repair: ERROR CODE P0300, emission control system, competent mechanic

Question
CHRYSLER 300M (2004). The car stop working so I took it to the agency, spent three weeks in repair and I have just had it back. Now a yellow light like an engine on the board appears, P0300 error. What could be? Tkanks in advance for your support.

Answer
Hi Lucy,
Can you tell me from the service receipt of that recent work what they said they did?
The 0300 means that there are misfirings on several "multiple" cylinders which suggests a common cause rather than an individual cylinder missing. Unfortunately there are a large number of possibilites which a competent mechanic should be able to eliminate and focus upon the one or two causes thay may be the problem. So I would go back and tell them that they didn't fix the problem and that they need to go over their work and see what is really/still wrong and that you believe that in light of the work that has been done at the cost involved that further analysis and repair should be covered under the warranty for that work. They didn't solve the problem so you shouldn't have to pay more for them to get it right.
Among the possible causes are: (though ideally they should have looked at most of them already): mixture-related issues such as the fuel pump or filter (unlikely as regards the filter), injector harness connectors, ignition coil circuit, mechanical problem (but compression test would probably have shown that), contaminated fuel, water in fuel. pcm power ground connections, injectors, restricted exhaust, intake restriction, the pcm itself, the evap emission control system, the exhaust gas recirc valve system (this is something I would certainly recommend that they check, as if the valve sticks ajar it will definitely cause misfiring. So suggest that they lube the valve stem with WD-40 and move it back and forth to be sure it is moving to a dead-stop via internal spring-action), air gap at high rpm's, leak at intake manifold, other mechanical (?). The parenthetical items are my remarks.
But the egr valve would be high on my list.
Good luck on this, and stand firm to insist that they correct the failed repair so far at no forther cost.
Roland