Mi toyota previa se le bajan las revolucioines y se apaga

SOURCE: mi dodge neon sxt 2.0Laut 04.me da un codigo p0300

Basically this means that the the car's computer has detected that not all of the engine's cylinders are firing properly.
A P0300 diagnostic code indicates a random or multiple misfire. If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A P0302 code, for example, would tell you cylinder number two is misfiring. Unfortunately, a P0300 doesn't tell you specifically which cylinder(s) is/are mis-firing, nor why.
Symptoms may include:

• the engine may be harder to start
• the engine may stumble / stumble, and/or hesitate
• other symptoms may also be present

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Causes A code P0300 may mean that one or more of the following has happened:

• Faulty spark plugs or wires
• Faulty coil (pack)
• Faulty oxygen sensor(s)
• Faulty fuel injector(s)
• Burned exhaust valve
• Faulty catalytic converter(s)
• Stuck/blocked EGR valve / passages
• Faulty camshaft position sensor
• Defective computer

Possible Solutions If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.
If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.
Random misfires that jump around from one cylinder to another (read: P030x codes) also will set a P0300 code. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open

SOURCE: quisiera que alguien me indicara

con el motor apagado apreta el boton para borrar el odometro (marcador de kilometros en el tablero) mantenlo apretado y sin soltarlo da contacto con la llave espera como 10 segundos hasta que la luz se apague

SOURCE: acaban de componer mi camioneta

Hola amigo, no se si ya resolviste tu problema,(espero), de otra forma, tendras que llevara tu camioneta con un mecanico electrico, pero que en verdad tenga conocimientos en este tipo de vehiculos, por que la gran mayoria de mecanicos, solo son "cambia piezas" y cuando desarman no es el problema, el problema es a la hora de armar, por que muchas veces ya no se acuerdan de como estaba antes, y pueden dejar algun arness flojo. en cuanto a esas fallas que comentas, al parecer es el sensor de velocidad,( lo mas probable es que no lo haya conectado bien;) Este sensor controla los cambios de la transmisioin y claro está tambien el velocimetro. saludos

SOURCE: tengo un 98 pontiac sunfire nadie le a encontrado

lo que tienes que cambiar es las bujias y los cables otra ves y asegurarte que esten bien apretadas las bujias y los cables oh otra cosa puede ser que tenga una fuga de gas en los cilindro pero asegurate que las bujias sean compatibles para esa marca