For 1990-2009 cars

The powertrain control module (PCM) uses pulses from the crankshaft position sensor to determine the position and RPM of the engine at various degrees of crankshaft rotation. The PCM bases misfire diagnostics on the principle that crankshaft velocity will increase as each cylinder contributes its power input. Under certain conditions, the PCM will monitor crankshaft velocity variation in a series of 200 revolution tests. If 20-100 percent of the revolutions fail, a catalyst damaging misfire has occurred. If 2-19 percent of the revolutions fail, an emission related misfire has occurred. The malfunction indicator light (MIL) (SERVICE ENGINE SOON telltale) will flash when an active catalyst damaging misfire occurs on the first drive cycle. The MIL will be ON steady if an emission related misfire occurs after two consecutive drive cycles. The MIL will turn OFF after three consecutive passed drive cycles meeting the same misfire criteria or 80 passed drive cycles if the criteria could not be met. DTC P0300 sets when a specific or random cylinder misfire has occurred.

### DTC Parameters

DTC P0300 will set if specific or random cylinder misfires have been detected after a series of 200 revolution tests when:

• | The engine speed is between 562-5500 RPM.

• | The engine coolant temperature (ECT) is greater than -7°C (20°F). If the ECT is less than -7°C (20°F), the PCM will wait until the ECT is greater than 20°C (68°F).

• | The ignition voltage is between 8-18 volts.

• | The fuel level is greater than 3.8 L (1 gallon).

• | Crank learn = YES. If crank learn = NO, engine run time must be greater than 20 seconds to enable misfire.

• | No 5 volt reference, cam, crankshaft position (CKP), ECT, fuel trim, idle air control (IAC) valve, intake air temperature (IAT), manifold absolute pressure (MAP), throttle position (TP) or vehicle speed DTCs have been set

DTC P0300 diagnostic runs continuously once the above conditions have been met.

DTC P0300 is a type B DTC.

### Diagnostic Aids

Important: If an ignition control (IC) circuit, fuel injector or exhaust gas recirculation (EGR) DTC is set, those should be diagnosed first because they may be the cause of the misfire.

If vehicle has had prior replacement of a PCM or crankshaft, the crankshaft must be relearned.

Use DATA DISPLAY information to check MISFIRE HISTORY CYL. #1-4 to determine whether the engine has a specific or random misfire.

Spraying water on wires provides a good path for current to flow to ground on an arcing wire.

Use a scan tool to monitor MISFIRE CURRENT CYL #1-4 while simulating different engine load conditions in the stall. The scan tool current misfire counter should pick up intermittent misfire conditions.

• | Fuel injector resistance: 12.2 +/- 0.4 ohms at 23°C (74°F)

• | Wire resistance: 1.5k-15k ohms

• | Spark plug gap: 0.040 in

• | Coil pack tower to tower secondary resistance: 6.5k-11.5k ohms

• | Wire position on coil packs

| 4-1-2-3

| Connector