Fault phenomenon: the Fiat 1.5L or the park sedan, which is partly equipped with a manual transmission, in the vehicle, step on the clutch pedal or skidding, and the engine speed rises to 2000 ~ 4000r/min when the throttle is loosened. Fault diagnosis: this phenomenon is usually caused by engine electronic control system failure. Fault reason: throttle dirty, will cause throttle door closed or idle speed stepping motor stuck, can cause engine idle speed too high. Troubleshooting method: you can remove the throttle cleaning, after installation must be adaptive. Adaptive method: install all parts and wiring, turn ignition switch to ON position to stay 10s. Turn off the ignition switch, re turn on the ignition switch and start the engine. Another possible reason: throttle position sensor failure. This fault is more obvious when the ambient temperature is high. The reason is that the internal components of the throttle sensor are unstable, and the wrong throttle opening signal is sent out when the temperature rises, which causes the engine idle speed to rise, and the engine speed can reach more than 4000r/min. The valve opening value can be read from the engine idling time by the tester, so as to assist in judging whether the throttle position sensor is malfunction. Method of exclusion: the throttle assembly must be replaced.