ck engine lite on, idles rough on 2002 Chrysler 300M

2002 chrysler 300m, getting code po300 multilpe miss fire, the customer has replaced the plugs/1 coil/oxygen sensor. when idling it starts shaking and acting like it wants to cut off. i was told these cars have a problem with carbon, and was told to use see foam thru vaccum port and let sit, then run the car steady for a period. did that and cleared the code and the car is running great the customer says. he had drove it about 40 miles and the ck engine light came back on, but it is still idling good. what could i ck

Asked by for the 2002 Chrysler 300M
You said it's a customer's car. I had a similar problem with a Nissan Altima, used Seafoam it was great. Have you looked on iATN, great resource for professional auto technicians. There is a professional service called Identafix. They only deal with professional repair shops. They are in the Minnesota 651-633-8007. You do all your diagnosis tests, and then discuss your problem with a team of professional technicians great service cheap and very bright people. This company only deal with automotive repair shops and don't answer questions for the general public but are an amazing knowledge base.
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Most likley the injectors are clogged or need replacing