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Q: Its misfiring on cyl 5 and has a code for an o2 sensor.

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Has been misfiring and running rough. The shop changed the timing belt, o2 sensor and mass air sensor, still did not fix.

My car has 188000 miles.
My car has an automatic transmission.

Diagnostic Trouble Code P0305 means that Cylinder No. 5 is misfiring but the causes of that can be very varied. Check the plug, secondary wire, and distributor cap. The fuel injector to the cylinder could simply be faulty or there might be low No. 5 cylinder compression due to a burned valve or leaky head gasket. Test for power TO the fuel injector coil as if there is no power to the coil the supply circuit (including the PCM) has to be diagnosed to find the fault. If there is power to the coil, I would check to see if the fuel injector is functioning and check compression. If you want these steps performed by a certified Mechanic, dispatched by YourMechanic right to your location, please request a misfiring diagnostic and the responding certified mechanic will get this diagnosed and resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic.

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