P0300

Circuit Description
The PCM has the ability to detect a misfire by monitoring the 3X reference and camshaft position input signals from the Ignition Control Module. The PCM monitors crankshaft speed variations (reference period differences) to determine if a misfire is occurring.
If 2% or more of all cylinder firing events are misfires, emission levels may exceed mandated standards. The PCM determines misfire level based on the number of misfire events monitored during a 200 engine revolution test sample. The PCM continuously tracks 16 consecutive 200 revolution test samples. If 22 or more misfires are detected during any 10 of the 16 samples, DTC P0300 will set. If the misfire is large enough to cause possible three-way catalytic converter damage, DTC P0300 may set during the first 200 revolution sample in which the misfire was detected. In the case of a catalyst damaging misfire, the MIL will flash to alert the vehicle operator of the potential of catalyst damage.

Conditions for Running the DTC
^ No VSS, Transmission, TP sensor, Fuel trim, Fuel injector, MAP sensor, EVAP, HO2S, IC/Bypass line monitor, EGR flow, EGR pintle position, ECT sensor, CKP sensor CMP sensor, or MAF sensor DTCs set.
^ Engine speed between 550 and 5900 RPM.
^ System voltage between 9 and 16 volts.
^ The ECT indicates an engine temperature between -6°C (21°F) and 120°C (248°F).
^ Throttle angle steady.

Conditions for Setting the DTC
The PCM is detecting a crankshaft RPM variation indicating a misfire sufficient to cause three-way catalytic converter damage or emissions levels to exceed mandated standard.

Action Taken When the DTC Sets
^ The PCM will illuminate the MIL during the second consecutive trip in which the diagnostic test has been run and failed.
^ The PCM will store conditions which were present when the DTC set as Freeze Frame and Fail Records data.

Conditions for Clearing the MIL/DTC
^ The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed.
^ The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC can be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed.

Diagnostic Aids
The scan tool provides information that can be useful in identifying the misfiring cylinder. If the DTC P0300 is currently stored as Test failed since code clear, the misfire history counters (Misfire Hist #1 - #6) will still contain a value that represents the level of misfire for each cylinder.
The scan tool displayed misfire counter values (Misfire Hist. #1 through #6) can be useful in determining whether the misfire is isolated to a single cylinder or to a cylinder pair (cylinders that share an ignition coil- 1/4, 2/5, 3/6.) If the largest amount of activity is isolated to a cylinder pair, check for the following conditions:
^ Secondary Ignition Wires: Check wires for affected cylinder pair for disconnected ignition wires or for excessive resistance. The wires should be 600 ohms per foot (1968 ohms per meter).
^ Damaged Or Faulty Ignition Coil: Check for cracks, carbon tracking or other damage. Also check coil secondary resistance. Secondary resistance should be between 5000 ohms and 7000 ohms (5K ohms and 7k ohms).
^ Substitute a Known Good Coil: Swap 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 crankcase ventilation valve, or for vacuum leaks at the throttle body, EGR valve, and intake manifold mounting surfaces.
^ Fuel Pressure: Perform a Fuel System Pressure Test. 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 Injector Coil and/or 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 rings.
^ EGR: Check for leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.

Reviewing the Fail 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.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table:
2. A malfunctioning injector circuit, crankshaft position system variation not learned condition, or incorrect rough road data from the EBCM may cause a misfire DTC to be set. If any of the indicated DTCs are set with DTC P0300, diagnose and repair the other DTC before using the DTC P0300 table.
3. The Misfire Current Cyl # display may normally display a small amount of activity (0 - 10 counts) but should not steadily increment during an entire 200 revolution test sample period.
4. Depending on the cause of the misfire, the Misfire History Cyl # counter will display a very large number for the misfiring cylinder(s); values for the non-misfiring cylinders will be less than 1/2 as great as the misfiring cylinder(s). When investigating a misfire, always start with items associated with the cylinder(s) that has the largest number of counts stored in the Misfire History Cyl # counter.
5. If the misfiring cylinders are companion cylinders, the condition is most likely linked to the ignition system.
9. Check for poor terminal connection, grooves, corrosion, pitting, loose fit.
17. Check for the following conditions that may contribute to the engine misfire: engine oil pressure, damaged accessory drive belt or pulley, damaged driven accessory (generator, water pump drive belt tensioner, etc.) Loose or broken motor mount(s).