Engine Performance problem
1996 Buick Lesabre 6 cyl Front Wheel Drive Automatic 58,555 miles
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This is my mother-in-law's car. It just had the intake manifold replaced at the dealer. ($863.19). We were driving it today and when I went to re-start the engine it back-fired thru the intake manifold and cut out. I started it again and the idle went up to 2,000 rpm and back down and ran bad. It surged at the stoplight and I had to slip it in neutral. On the highway it did not have the power when you push down the gas pedal.

I put my code scanner on it and it read PO121. - TPS/Pedal Position Sensor A CKT Range/Perf.

How can I fix it?