2002 Toyota RAV4
P0304 is a misfire on Cylinder #4, so either a faulty spark plug or coil in the cylinder and in some rare cases the piston or valve is damaged. If the plugs are original, change them out. If you still get the code, then it could be the coil, swap the coils to another cylinder to see if it moves. Then you know it's the coil or not. A P0171 usually indicates a vacuum leak. Check your air hoses for cracks or splits.
Thank you for responding. I will try that.
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Mandy Crowe Johnson