Diagnostic Trouble Code P0308 Definition, Possible Causes, & Diagnosis

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Diagnostic Trouble Code P0308 Definition

P0308 indicates an engine misfire on cylinder 8.

The diagnostic trouble code (DTC) P0308 indicates an engine misfire on cylinder 8 and sets when the engine control module (ECM) detects an excessive number of engine misfires on cylinder 8 over a given period of time. The ECM detects engine misfires by monitoring for variations in crankshaft speed, indicated by the signal from the crankshaft position sensor. When a cylinder misfires, a slight drop in crankshaft speed occurs. This speed variation along with the rotational position of the crankshaft allows the ECM to determine that a misfire occurred as well as which cylinder misfired. When the ECM registers an excessive number of misfires on cylinder 8, it sets P0308 and illuminates the check engine light.

P0308 Possible Causes

Before determining possible causes of P0308, it is critical to check for any additional engine DTC's and review the misfire counts for all cylinders using a capable scan tool. Its important to note and understand any other engine DTC's that are set as they can point to the root cause of P0308. For example, if we had a P0171 set for a lean condition along with P0308, we would want to diagnose the lean condition first as it is likely what is causing the misfires. Other examples of DTC's that should be prioritized over P0308 include any codes related to fuel pressure, camshaft timing, fuel injector control, and ignition coil control. Additionally, other single cylinder misfire DTC's (P0301, P0302, etc.) or P0300 (multiple cylinder misfire) indicate that the issue is not isolated to cylinder 8 and should be diagnosed as a multiple cylinder misfire. Be sure to fully understand all engine DTC's that are set when determining whether or not P0308 is the priority DTC.

Here are some examples of possible causes of P0308:

• Spark plug

• Ignition coil

• Fuel injector

• Engine mechanical (low compression, valve spring, camshaft lobe, head gasket, etc.)

• Intake leak

• Ignition coil or fuel injector circuits

• ECM

P0308 Diagnosis

Here are some general steps to aid in the diagnosis of P0308:

Inspect the condition of related electrical connectors: Loose connections at the ignition coil, fuel injector, ECM, and wire-to-wire connectors can result in misfires and P0308.

Inspect the condition of the spark plugs and ignition coils on the misfiring cylinders: Replace as needed. Spark plugs and ignition coils can also be swapped between cylinders to see if the misfires follow either component. If ignition coils are found to be burnt or swollen, test the ignition coil ground circuit for an excessive voltage drop.

Perform a compression test: Low compression due to an engine mechanical issue is a common cause of misfires. If low compression is found, perform cylinder leakage testing to determine the cause of the low compression.

Scope the ignition coil and fuel injector control circuits: Compare the scope patterns on cylinder 8 to the other cylinders. An incorrect scope pattern can point to an ECM, circuit, ignition coil, or fuel injector issue.

Monitor engine vacuum during the misfire: If the vacuum gauge reading is erratic, a valvetrain issue is indicated. If that is the case, inspect the cylinder 8 camshaft lobes, valve springs, and lifters; check for any excessive wear or damage.