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Here are some things to consider investigating:
(1)
The Flashing CEL Demands an immediate solution. Continuing to Drive the SUV can Ruin the CAT.
(2) Carefully R&R the MAP Sensor from the Rear Intake Manifold and Clean out any Oil Build Up.
(3) As a matter of course... R&R and Clean the Throttle Body.
(4) Carefully go over the Engine Bay and examine ALL Vacuum Hoses for Looseness & Cracking.
(5) Change out the Air Filter. When Extremely Dirty, Air Starvation can affect Stoich Balance.
(6) A Clogged CPAS (Screens Clogged) can cause Random Misfires. This Video shows how to change it out.
The Motor will also need an Engine Flush and Fresh Oil & Oil Filter Change.
(6) On Early Model TBs & Envoys, Change the Fuel Filter as per
@MAY03LT 's Instructional Video:
(7) On Early Model TB, thankfully you can also attach an Analog Fuel Pressure Gauge and Test Fuel Pressure at a "Bone Idle" of 600 RPM. A Sketchy Fuel Pump can also be a cause for the random lean-out conditions presented with throwing a P0300 Code. This Video from
@MAY03LT is the perfect instructional for this FP Test:
Please avoid purchasing any other Components until you can work through these and other better suggestions from our Members and sort things out. Work any additional PXXXX Codes being held in the PCM Memory in their Order of Appearance.