P0300-P0399:

Diagnosing Misfires

Misfire is a common driveability problem that may or may not be easy to diagnose, depending on the cause. A misfiring cylinder in a four-cylinder engine is, pardon the pun, hard to miss. The loss of 25% of the engine’s power output is the equivalent of a horse trying to run on three legs. The engine may shake so badly at idle that it causes vibrations that can be felt in the steering wheel and throughout the vehicle. The engine also may be hard to start and may even stall at idle, depending on the accessory load (air conditioning, headlights and electric rear defroster, for example).

When misfire occurs, performance suffers along with fuel economy, emissions and idle quality. And, when a misfiring vehicle is subjected to an emissions test, it will usually fail because of the unusually high levels of hydrocarbons (HC) in the exhaust. What causes a cylinder to misfire? Basically, it’s one of three things: loss of spark; the air/fuel mixture is too far out of balance to ignite; or loss of compression. Loss of spark includes anything that prevents coil voltage from jumping the electrode gap at the end of the spark plug. Causes include worn, fouled or damaged spark plugs, bad plug wires or even a cracked distributor cap. A weak coil or excessive rotor gas inside a distributor would affect all cylinders, not just a single cylinder.

“Lean misfire” can occur when the air/fuel mixture is too lean (not enough gasoline in the mixture) to burn. This can be caused by a dirty, clogged or inoperative fuel injector; air leaks; or low fuel pressure because of a weak pump, restricted filter or leaky pressure regulator. Low fuel pressure would affect all cylinders rather than an individual cylinder, as would most air leaks. A leaky EGR valve can also have the same effect as an air leak. Loss of compression means the cylinder loses most of its air/fuel mixture before it can be ignited. The most likely causes here are a leaky (burned) exhaust valve or a blown head gasket. If two adjacent cylinders are misfiring, it’s likely the head gasket between them has failed. Also, if an engine is overheating or losing coolant, it’s likely the head gasket is the culprit. Intermittent misfires are the worst kind to diagnose because the misfire comes and goes depending on engine load or operating conditions. They seem to occur for no apparent reason. The engine may only misfire and run rough when cold but then smooth out as it warms up. Or, it may start and idle fine but then misfire or hesitate when it comes under load. Also, it may run fine most of the time but suddenly misfire or cut out for no apparent reason. Intermittent misfires can be a real challenge to diagnose, so let’s start with a steady misfire in one cylinder before moving on to intermittent misfires.

Steady Misfire

In the case of a steady misfire, isolating the misfiring cylinder is the first step in diagnosing the problem. Today’s OBDII systems make this easy using a scantool such as TOAD. Simply use TOAD to read the Diagnostic Trouble Code (DTC) stored in the PCM. This is preferable to the traditional method of pulling plug wires to identify the weak cylinder, because it prevents the voltage from causing any damage to the electronics in the ignition system. When a plug wire is physically disconnected from a spark plug, the high voltage surge from the coil cannot follow its normal path to ground through the plug wire and spark plug, so it passes back through the coil. Most ignition systems are robust enough to withstand such voltage backups intermittently but not on a prolonged basis. If the coil or ignition module is already weak, it may push the component over the brink causing it to fail.

  • P0300       Random/Multiple Cylinder Misfire Detected
  • P0301       Cylinder 1 Misfire Detected
  • P0302       Cylinder 2 Misfire Detected
  • P0303       Cylinder 3 Misfire Detected
  • P0304       Cylinder 4 Misfire Detected
  • P0305       Cylinder 5 Misfire Detected
  • P0306       Cylinder 6 Misfire Detected
  • P0307       Cylinder 7 Misfire Detected
  • P0308       Cylinder 8 Misfire Detected
  • P0309       Cylinder 9 Misfire Detected
  • P0310       Cylinder 10 Misfire Detected
  • P0311       Cylinder 11 Misfire Detected
  • P0312       Cylinder 12 Misfire Detected
  • P0320       Ignition/Distributor Engine Speed Input Circuit Malfunction
  • P0321       Ignition/Distributor Engine Speed Input Circuit Range/Performance
  • P0322       Ignition/Distributor Engine Speed Input Circuit No Signal
  • P0323       Ignition/Distributor Engine Speed Input Circuit Intermittent
  • P0325       Knock Sensor 1 Circuit Malfunction (Bank 1 or Single Sensor)
  • P0326       Knock Sensor 1 Circuit Range/Performance (Bank 1 or Single Sensor)
  • P0327       Knock Sensor 1 Circuit low Input (Bank 1 or Single Sensor)
  • P0328       Knock Sensor 1 Circuit High Input (Bank 1 or Single Sensor)
  • P0329       Knock Sensor 1 Circuit Input Intermittent (Bank 1 or Single Sensor)
  • P0330       Knock Sensor 2 Circuit Malfunction (Bank 2)
  • P0331       Knock Sensor 2 Circuit Range/Performance (Bank )
  • P0332       Knock Sensor 2 Circuit Low Input (Bank 2)
  • P0333       Knock Sensor 2 Circuit High Input (Bank 2)
  • P0334       Knock Sensor 2 Circuit Input Intermittent (Bank 2)

The crankshaft position sensor generates a timing signal for the ignition system by reading notches in the crankshaft balancer, flywheel or crankshaft. Two types are used: magnetic and Hall effect. Magnetic sensors produce an alternating current (AC) signal that changes in frequency and amplitude with rpm. Hall effect switches produce an on-off digital signal. The powertrain control module (PCM) may modify the timing signal from the sensor to advance or retard ignition timing. Loss of the sensor signal will prevent the engine from starting (no spark). A “cranks but won’t start” condition may be due to a bad crankshaft (or camshaft) position sensor. If the sensor is shorted, open or out of range, the PCM can’t get a good crank position signal to trigger the ignition coils. NOTE: A magnetic crankshaft position sensor can be checked with an ohmmeter. Replace the sensor if the resistance reading is out of range (typically around 700 ohms). ALSO NOTE: Some crank position sensors may test okay at room temperature, but go open or short when they get hot. An intermittent cranks but won’t start condition may be caused by this kind of problem.

  • P0335       Crankshaft Position Sensor A Circuit Malfunction
  • P0336       Crankshaft Position Sensor A Circuit Range/Performance
  • P0337       Crankshaft Position Sensor A Circuit Low Input
  • P0338       Crankshaft Position Sensor A Circuit High Input
  • P0339       Crankshaft Position Sensor A Circuit Intermittent
  • P0340       Camshaft Position Sensor Circuit Malfunction
  • P0341       Camshaft Position Sensor Circuit Range/Performance
  • P0342       Camshaft Position Sensor Circuit Low Input
  • P0343       Camshaft Position Sensor Circuit High Input
  • P0344       Camshaft Position Sensor Circuit Intermittent

When a coil failure occurs on a distributor ignition system, it affects all the cylinders. The engine may not start or it may misfire badly when under load. But with multi-coil ignition systems, a single coil failure will only affect one cylinder (or paired cylinders in the case of waste spark DIS systems). On 1996 and newer vehicles, the OBD II system should detect coil problems as well as misfires and generate a fault codes that identify the problem coil or cylinder. A misfire code P0301, for example, would indicate a misfire problem in cylinder #1. Of course, misfires can be caused by a lot of things. It could be a worn or fouled spark plug, a weak coil, a bad plug wire or connection in the case of a DIS or CNP system, a dirty or dead fuel injector, or a loss of compression (burned exhaust valve or leaky head gasket). Further diagnosis is always needed to isolate and identify the cause — which creates a problem on multi-coil systems that do not have spark plug wires because you can’t observe the secondary ignition pattern unless you use some type of adapters or inductive pickups that fit on the coils themselves.

  • P0350       Ignition Coil Primary/Secondary Circuit Malfunction
  • P0351       Ignition Coil A Primary/Secondary Circuit Malfunction
  • P0352       Ignition Coil B Primary/Secondary Circuit Malfunction
  • P0353       Ignition Coil C Primary/Secondary Circuit Malfunction
  • P0354       Ignition Coil D Primary/Secondary Circuit Malfunction
  • P0355       Ignition Coil B Primary/Secondary Circuit Malfunction
  • P0356       Ignition Coil F Primary/Secondary Circuit Malfunction
  • P0357       Ignition Coil G Primary/Secondary Circuit Malfunction
  • P0358       Ignition Coil H Primary/Secondary Circuit Malfunction
  • P0359       Ignition Coil I Primary/Secondary Circuit Malfunction
  • P0360       Ignition Coil I Primary/Secondary Circuit Malfunction
  • P0361       Ignition Coil K Primary/Secondary Circuit Malfunction
  • P0362       Ignition Coil L Primary/Secondary Circuit Malfunction
  • P0370       Timing Reference High Resolution Signal A Malfunction
  • P0371       Timing Reference High Resolution Signal A Too Many Pulses
  • P0372       Timing Reference High Resolution Signal A Too Few Pulses
  • P0373       Timing Reference High Resolution Signal A Intermittent/Erratic Pulses
  • P0374       Timing Reference High Resolution Signal A No Pulses
  • P0375       Timing Reference High Resolution Signal B Malfunction
  • P0376       Timing Reference High Resolution Signal B Too Many Pulses
  • P0377       Timing Reference High Resolution Signal B Too Few Pulses
  • P0378       Timing Reference High Resolution Signal B Intermittent/Erratic Pulses
  • P0379       Timing Reference High Resolution Signal B No Pulses
  • P0380       Glow Plug/Heater Circuit “A” Malfunction
  • P0381       Glow Plug/Heater Indicator Circuit Malfunction
  • P0382       Glow Plug/Heater Circuit “B” Malfunction
  • P0385       Crankshaft Position Sensor B Circuit Malfunction
  • P0386       Crankshaft Position Sensor B Circuit Range/Performance
  • P0387       Crankshaft Position Sensor B Circuit Low Input
  • P0388       Crankshaft Position Sensor B Circuit High Input
  • P0389 Crankshaft Position Sensor B Circuit Intermittent