Error Code P0305 is described as Misfire Detected in Cylinder #5, which means the vehicle’s computer has determined #5 cylinder that is not firing properly.


Error Code P0305 is one of the most common error codes, as engine misfire is a quite a common problem in vehicles. This code signifies misfire problem on cylinder #5. Modern vehicles (vehicles made 1996 on wards) usually are equipped with a computer system called Power train Control Module (PCM), which detects engine troubles, including misfires detection. As soon as it detects the issue, it then automatically triggers the Check Engine light on.

Engine misfire can be caused by a lot of things, such as lack of spark, low compression, poor fuel quality, metering, unmetered air entering the engine, etc., which leads to lack of combustion. When this happens, the computer triggers the Check Engine light and shows a code such as P0305. The last digit of this number points out the exact cylinder at fault. For example, if the Check Engine light comes with a number combination of P0301, that means the cylinder #1 is misfiring. P0302 means misfire on cylinder #2 and so on.

Common Symptoms

  • Check Engine light is On
  • The vehicle is harder to start than usual
  • Stumbles or hesitates upon acceleration
  • In most cases, the vehicle will demonstrate performance problems like lack of power on acceleration, dying at stop signs, rough riding, hesitation and bad fuel consumption.

Possible Causes

  • Problems on the Evaporative Emission System (EVAP), i.e. vapor leak, which results to the engine
  • Exhaust Gas Recirculation (EGR) problems, i.e. restricted ports, EGR valve leak
  • Running out of gas or poor fuel quality
  • Defective MAF (Mass Air Flow) sensor
  • Wrong throttle position
  • Base ignition problem such as low compression, bad timing, and valve train issues
  • Ignition system issue such as defective spark plug, wiring related problems, ignition module and wiring problems, and ignition sensor or wiring issues
  • Faulty Camshaft sensor or Crankshaft sensor
  • Faulty computer system

How To Check

When Error Code P0305 appears with the Check Engine light, one of the first things you need to do is to duplicate the conditions that set the code through a test drive. This means taking note of the freeze frame information, including the engine load, road speed, throttles position and RPM. The error code refers to a specific cylinder misfire, which means it needs a more thorough diagnosis.

For a cylinder misfire, you have to change faulty components, such as coils, spark plugs, etc. This is to isolate the exact cause of the misfire. Also, it is crucial to note other error codes, as the misfire may result from other faulty components.

Here are four common types of engine misfire and how to check for the error code P0305.

Ignition Misfire

Check the Ignition system and all connected components for signs of heat damage and wear and tear. A spark plug that is in good condition must not be blackened with soot, whitened by overheating combustion chamber or greened by the coolant. Rather, it should maintain a sandy color.

Cables and coils of the ignition system should be free of arcing. Scope the Ignition System if necessary; make sure it is firing even voltages, which should be about 8V to 10V for every cylinder.

Remove the distributor cap and rotor (if necessary) from the engine. Check the contact points and terminals for signs of wear and tear, corrosion build up or arching.

Though OBD II vehicles usually comes with computer-controlled trimming, make sure it’s the right specifications, even if it is using individual coils.

Lean Misfire

To check for lean misfire, take note of the Long Fuel Trim values, as it determines how the PCM (Power train Computer) works to compensate the balance of the air/fuel ratio. If result shows more than 10% in a one cylinder bank and not on the other, this can be a sign of vacuum leak, crack or defect on the manifold on the specified bank. You will know what causes the computer to do the compensation by checking the “numbers” of the Fuel Trim from the full range of operating conditions. Vehicle engines that are in good shape have around 1% to 3% Long Term Fuel Trim numbers, either positive or negative.

Mechanical Misfire

There are basic two ways to check for this misfire: Compression test and Engine Idle Manifold Vacuum test. For a complete and smooth combustion, there should be a consistent compression readings (about 10% of each other), a minimum of 120 PSI for every cylinder and with at least 17-inches of steady vacuum.

Powertrain Misfire

Vehicles may trigger this error code when badly warped and out-of-bound rear brake drums jerk the whole power train violently while the vehicle slows from highway speeds. To diagnose the vehicle for this misfire, it is important to bring your vehicle to a professional for a more thorough inspection and pinpoint the cause of the misfire. In some cases, wrongly perceive mechanical misfires that starts from the driveshaft, transfer case, transmission or front/rear differential results to replacement of the whole engine.

How To Fix

For vehicles that don’t show much symptoms, a simple reset of the code and retest will get rid of the error code.

If a couple or more symptoms are present though, such as engine stumbling or hesitation, you must check for damages on the wiring and connectors to the cylinder, such as the spark plugs.

Ignition components must be regularly maintained, or even replaced, as part of a regular maintenance check. This includes the distributor cap, wires, rotor (if applicable) and spark plugs. Otherwise, you should inspect the coil (coil packs).

If you notice smell of rotten egg from the exhaust, then that’s a tell-tale sign of damage in the catalytic converter, and it means it needs replacement.

In some rare cases, the error code may be triggered by faulty fuel injectors.

Parting Tips

For more information about this error code and other engine misfire error codes, feel free to browse through our website. For professional mechanical help, Contact Us.