How do I fix my car its got P0300 code and idles.

DTC P0300 Engine Misfire Detected

DTC P0300 Engine Misfire Detected Circuit Description
The powertrain control module (PCM) is able to detect a misfire by monitoring the 58X reference and the camshaft position input signals. If the PCM detects crankshaft speed variations that indicate 1% or more of cylinder firing events are misfires, the PCM will disable the torque converter clutch (TCC). If the RPM variation detected indicate a true misfire condition, DTC P0300 will be set.

Conditions for Setting the DTC

None of the following DTCs occur: TP sensor, MAF sensor, CMP sensor, VSS, ECT sensor, ABS Rough Road sensor, CKP sensor.
The engine speed is between 600 and 6250 RPM.
The system voltage is between 11 and 16 volts.
The engine temperature sensor (ECT) indicates an engine temperature between -7 °C (20 °F) and 120 °C (248 °F).
Throttle angle is steady and throttle changes less than 3% per 125 milliseconds.
The PCM detects a crankshaft RPM variation indicating a misfire that is sufficient to cause catalytic converter damage or emissions levels to exceed the mandated standard.

Action Taken When the DTC Sets

The PCM will illuminate the malfunction indicator lamp (MIL) the first time the fault is detected.
If the misfire is severe enough to cause possible catalyst damage, the PCM will flash the MIL for as long as the misfire remains at catalyst damaging levels.
The PCM will disable the TCC operation.
The PCM will store conditions which were present when the DTC was set as Freeze Frame and in the Failure Records data.

Conditions for Clearing the MIL/DTC

The PCM will turn the MIL OFF on the third consecutive trip cycle in which the diagnostic has been run and the fault condition is no longer present.
A history DTC P0300 will clear after 40 consecutive warm-up cycles occur without a fault.
DTC P0300 can be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed.

Diagnostic Aids
The scan tool display Misfire Cur. #1 through #6 can be useful to determine whether the misfire is isolated to a single cylinder.

Damaged or faulty ignition coil - Check for cracks or other damage.
Substitute a known good coil - Swap the ignition coils and retest. If the misfire follows the coil, replace the ignition coil. If the misfire is random, check for the following conditions:
System grounds - Ensure all connections are clean and properly tightened.
MAF - A mass air flow (MAF) sensor output that causes the PCM to sense a lower than normal air flow will cause a lean condition.
Air induction system - Air leaks into the induction system which bypass the MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, incorrectly installed or faulty PCV valve, or for vacuum leaks at the throttle body, EGR valve, and intake manifold mounting surfaces.
Fuel pressure - Perform a fuel system pressure test. A faulty fuel pump, plugged filter, or faulty fuel system pressure regulator will contribute to a lean condition.
Injector(s) - Perform an injector coil/balance test to locate faulty injector(s) contributing to a lean or flooding condition. In addition to the above test, check the condition of the injector O-ring.
EGR - Check for a leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.
Fuel quality - Using fuel with the wrong octane rating for the vehicle may cause driveability problems. Although alcohol-enhanced fuels may raise the octane rating, the fuel's ability to turn into vapor in cold temperatures deteriorates. This may affect the cold driveability of the engine. The Reid Vapor Pressure of the fuel can also create problems in the fuel system, especially during the spring and fall when changes by the refineries may not coincide with changes in the weather.
Vehicle marshalling - The transportation of new vehicles from the assembly plant to the dealership can involve as many as 60 key cycles within 2 to 3 miles of driving. This type of operation contributes to the fuel fouling of the spark plugs and will turn on the MIL with a P0300 Misfire DTC.

Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.