The reason why you car runs better after resetting the SES is that if there are misfires, the computer will turn off the injectors to the cylinders with the misfires. The misfires may be a slight misfire such as a weak spark or a lean condition but after the computer has detect x amount of misfires it will shut off the injector to prevent damage to the cat converter. Once the injectors are turn off that is a big lost in power because the cylinders are basically dead, whereas before they were still contributing power but not as much power.

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2006 Infiniti G35
2001 BMW 3.0I E53 X5 Build date 08/2000 SOLD
Lotus Europa 1970 Destroyed by fire
Lotus Europa 1970 S2 Renault Powered
Lotus Type 52 1970 Twincam Webers Powered
PORSCHE 911 Targa 1982 The Garage Queen
Audi Avant donated to Kars for Kids
BMW 525IT Sold
Audi 4000CS Quattro Sold
Jensen Healey Lotus Powered Sold
Opel 1900 Sold
Triumph Spitfire 1971 Sold
Triumph Spitfire 1968 Sold
Plymouth "Cuda" 340 Six pack SOLD