Spark Plug Failure Diagnosis Chart


Post created: 21 June 2021

Spark plugs are one of the most misunderstood components of an engine. Which is why AutoZone has created this Diagnostic Guide to help the technician, hobbyist, or race mechanic in understanding and troubleshooting spark plugs. The information stipulated in this guide applies to all types of internal combustion engines, including two stroke engines, rotary engines, high performance/racing engines and street vehicles.

The spark plug has two fundamental functions which are as follows:

  1. To ignite the air/fuel mixture at the correct time in the relevant cylinder
  2. To dissipate heat from the combustion chamber valve region in the head

Essentially, spark plugs transmit electrical energy that turns fuel into working energy. An adequate amount of voltage must be supplied by the ignition system to cause it to spark across the spark plug’s gap (centre electrode and side electrode), this is called “Electrical Performance”. When your spark plugs malfunction, it can cause many issues with the vehicle. Mainly your engine misfires, your car battles to start, you’ll notice excessive exhaust smoke from the vehicle’s exhaust tail pipe mainly due to unburnt/ incorrectly ignited fuel in the piston / sleeve area. This is also mainly due to incorrect volumes of fuel where and when the vital ignition is required. Here are some of our most common diagnoses for spark plug faults which you might experience:

Difficulty starting Misfiring Black exhaust smokeRetarded ignition Low compression Spark plug heat range is too low Spark plug ignition timing is incorrect (either to soon or after spark plug ignition).Check fuel float level (specific to carburettors) Check choke (specific to carburettors) Check ignition timing Check air filter Check compression Replace spark plug with correct heat range as per vehicle’s recommendations
Difficulty starting Misfiring Grey/white exhaust smoke Loss of oil/smoking after extended period of idlingWorn rings/low compression Worn piston to cylinder clearances Leaking valve stem seals Overfilled oil capacity in sumpChange worn parts Replace spark plug with correct heat range
Knocking sound in the engine when accelerating /under engine load or climbing hills Engine run-on after switching offLean mixture Advanced ignition timing Spark plug heat range is too highCheck carburettor fuel jets aren’t obstructed Check float level Check ignition timing Replace spark plug with correct heat range as specified
Misfiring Loss of power Difficulty starting Noise in engineForeign particles excessive carbon build up inside cylinders Broken or damaged valve Incorrect valve seating resulting in not sealing 100%Replace spark plugs Remove damaged internal components or fouled plugs
Melted spark plug Damaged piston crown Damage to cylinder headSpark plug torqued incorrectly Spark plug only tightened by hand Dirt or carbon in threads of cylinder heads Incorrect heat range of spark plugReplace spark plugs Tighten spark plugs to correct torque Replace damaged components Check compression pressures on all cylinders
Difficulty starting Reduced fuel economyNormal electrode wear Fuel deposits of unburnt fuel (Carbon build up, lead, additives and oil build up)Replace spark plug with the correct heat range
Red/brown corrosive stains above metal shell on the spark plug’s ceramic insulator.Oil particles dispersed in the air stick to the insulator as a result of high voltage.Change spark plug only at recommended service intervals as recommended by the manufacturer

Be sure to visit us in-store for spark plugs from brands like NGK, Bosch and Champion. Should you need further advice on spark plug diagnoses or simply need new spark plugs and engine components, give our AutoZone Experts a call on 086 112 2111 or email us at