How to check the spark plugs on a car yourself: 5 working methods

Spark plugs are small but critical engine parts, on which engine stability, fuel consumption and even acceleration dynamics depend. According to statistics, up to 30% of engine starting problems are associated with faulty spark plugs, but many car owners ignore checking them until the last minute. Meanwhile, you can diagnose the condition of the spark plugs yourself, without resorting to the help of a service station - you just need to know a few simple methods and have a minimal set of tools.

In this article we will look at 5 proven ways to check spark plugs - from visual inspection to testing on a specialized stand, and we will also tell you what signs of trouble should alert the driver. You will learn how to properly unscrew spark plugs so as not to damage the threads, what tools are needed for diagnostics, and when it is better to immediately replace the set without wasting time on checks. We will pay special attention a common mistake with “cleaning” spark plugs with sandpaper - why this should not be done and how it threatens the engine.

Signs of Bad Spark Plugs: When to Check

The engine is running rough and there is a light on the dashboard Check Engine? This is a reason to check the spark plugs first. But there are also less obvious symptoms, which many attribute to “bad gasoline” or “cold weather.” Pay attention to the following signals:

• 🔥 "Trippling" of the engine — the engine vibrates at idle, misfires are heard (especially noticeable on 4-cylinder engines).
• ⚡ Deterioration in dynamics — the car accelerates poorly, becomes “stupid” when you press the gas, and loses power at high speeds.
• 💨 Increased fuel consumption — if you fill up more often than usual without objective reasons (for example, in winter), the spark plugs may be to blame.
• 🔊 Popping sounds in the exhaust system — unburned fuel burns out in the outlet, which is accompanied by loud bangs.
• 🚗 Startup problems — the engine starts after 3-5 attempts, especially when it’s cold or after a long period of parking.

It is important to understand that these symptoms may also indicate other problems (for example, faulty ignition coils, breakdowns in high-voltage wires or dirty injectors). However checking spark plugs is the fastest and cheapest way to exclude one of the most likely causes. If the spark plugs are in order, you can move further along the diagnostic chain.

⚠️ Attention: On modern vehicles with the system EFI (Electronic Fuel Injection) faulty spark plugs may not produce obvious symptoms until they fail completely. Electronics compensate for misfires, masking the problem. Regular checks (every 30,000 km) are mandatory!

Preparing for an inspection: tools and safety measures

Before you start diagnosing, prepare everything you need. Minimum set of tools:

• 🔧 Candle key with a rubber seal (so as not to damage the insulator) or a head with an extension.
• 🔌 Multimeter (to check resistance and voltage).
• 🔥 "Probe" for spark (you can make it yourself from an alligator clip and wire).
• 🧴 Compressor or compressed air cylinder (for cleaning wells from dirt).
• 📸 Smartphone - to photograph the location of high-voltage wires (if they are not marked).

You will also need gloves (to avoid getting your hands dirty and to avoid cuts on the sharp edges of the block head) and flashlight - candle wells are often poorly lit. If you plan to check the spark, prepare spare spark plug (in case the one being tested turns out to be faulty).

⚠️ Attention: Never unscrew spark plugs hot engine! Risk of getting burned, damaging the threads in the block head or breaking the insulator. Let the engine cool to a temperature of 40-50°C (you can touch the air filter housing with your hand - it should not burn).

Before starting work disconnect the negative terminal of the battery - this will protect you from an accidental short circuit when checking the spark. If your machine has custom ignition coils (one per candle), remove them carefully, pulling up without distortion. Disconnect high-voltage wires on older systems by holding the cap, not the wire!

Method 1: Visual inspection of spark plugs - what carbon deposits can tell you

The simplest but most informative diagnostic method is inspection of electrodes and insulator. The color and structure of soot can tell a lot not only about the condition of the spark plugs, but also about the operation of the engine as a whole. Unscrew the spark plugs (one at a time, so as not to mix up the cylinders) and compare them with reference photographs.

Soot color	Reason	What to do
Light gray or brown (uniform)	Normal condition. The spark plug works correctly and the air-fuel mixture burns completely.	Can be installed back. Check the electrode gap (must comply with manufacturer's recommendations).
Black velvety (dry)	Rich air-fuel mixture (lots of gasoline, not enough air). Causes: dirty air filter, faulty injectors or oxygen sensor.	Check Mass air flow sensor, oxygen sensor, air filter. There may be a vacuum leak in the intake manifold.
White or gray (with metallic inclusions)	Lean mixture (too much air, not enough fuel) or spark plug overheating. Reasons: air leaks, faulty thermostat, too “hot” spark plugs (wrong heat number).	Check thermostat, cooling system, selection of spark plugs according to heat rating. Risk of valve burnout!
Reddish or brick	Deposits from additives in fuel (especially after refueling with “octane number improvers”).	Replace spark plugs, change gas station. If the carbon deposits do not disappear, flush the fuel system.
Oily black (with drops of oil)	Oil entering the combustion chamber. Reasons: worn valve stem seals, stuck piston rings, problems with PCV valve.	Engine diagnostics required. Replacing spark plugs is a temporary solution.

Pay special attention condition of the electrodes:

• 🔍 Worn or rounded electrodes - a sign of normal wear and tear. The spark plugs have exhausted their service life (usually 30-100 thousand km depending on the type).
• 💥 Melted electrodes - indicate detonation or overheating. Possible causes: incorrect fuel octane number, early ignition, faulty knock sensor.
• 🛑 Cracks on the insulator - The spark plug must be replaced. May lead to a spark breaking through to the housing.

Important: Never clean spark plugs with sandpaper or a wire brush! This removes the special coating on the electrodes (for example, iridium or platinum), accelerating their wear. For cleaning, use only chemical products (for example, carburetor cleaner) or an ultrasonic bath.

How to properly clean carbon from candles?

To clean, immerse the spark plugs in a solution of carburetor cleaner (for example, ABRO CC-220) for 30-40 minutes, then rinse with water and dry. An alternative is an ultrasonic bath (used at service stations). Mechanical cleaning with abrasives is prohibited!

Method 2: Checking the spark on a unscrewed spark plug (with video)

If the visual inspection reveals no obvious problems, the next step is checking for spark. This method is suitable for older distributor ignition systems (distributor) or for individual coils. Important: the test is carried out only on known good spark plug (you can use a new one or a known working one).

Verification algorithm:

1. Unscrew the spark plug from the cylinder (for example, the first one).
2. Place a high-voltage wire on it or install an individual coil.
3. Press the candle skirt firmly against unpainted metal part of the engine (for example, to the valve cover) through a dielectric gasket (a rubber hose can be used).
4. Have a helper crank the engine using the starter (or use the remote starter if equipped).

The spark should be:

• 🔥 Stable — no skips when cranking the starter.
• 💙 Blue color - This is a sign of high spark energy. A red or orange spark indicates poor ignition.
• 📏 Long — breakdown should occur at a distance of at least 3-5 mm (depending on the type of candle).

Video instruction: See what a normal spark should look like (example for VAZ 2110 with distributor):

[VIDEO: Checking the spark on the spark plug - normal vs malfunction]

⚠️ Attention: Do not hold the candle in your hands while checking! The spark discharge can be up to 40,000 volts - this is life-threatening. Always use insulated tools and dielectric gloves.

If there is no spark:

• 🔌 Check high voltage wire for a break (with a multimeter in resistance mode - it should be 3-10 kOhm).
• 📱 Check ignition coil (primary winding resistance: 0.5-2 Ohm, secondary: 6-15 kOhm).
• 🔧 Check distributor (on old cars) or ignition module.

Method 3: Multimeter Test - Resistance and Voltage

A multimeter will help you evaluate condition of the internal elements of the candle, but will not give a complete picture (for example, it will not show the quality of the spark). However, this is a quick way to weed out obviously faulty instances.

Resistance test:

1. Set the multimeter to resistance measurement mode (20 kOhm).
2. Connect the probes to the center electrode and the contact on the spark plug skirt.
3. Compare the readings with the reference ones for your model (usually 5-15 kOhm for resistor spark plugs).

The absence of resistance (or the value "1" - open) indicates a malfunction of the built-in resistor. This spark plug must be replaced.

Checking the breakdown voltage (for experienced ones):

This test requires special spark tester or a megohmmeter, but you can get by with a multimeter with a diode testing function:

1. Set the diode "testing" mode.
2. Connect the probes to the center and side electrodes.
3. If the device shows “0”, there is a breakdown in the insulator (the spark plug is faulty).

For accurate diagnosis it is better to use specialized spark plug tester (for example, Bosch FSA 050 or Autel MS300). It simulates operating conditions in the engine and shows the actual breakdown voltage.

Method 4: Replacement test - how to identify a faulty cylinder

If in the engine troit (works unevenly), but all candles look normal in appearance, use the rearrangement method. It helps determine whether the problem is with the spark plug or with another element (coil, wire, injector).

Algorithm:

1. Start the engine and remember which cylinder is “troubling” (you can disconnect the high-voltage wires one by one - when you disconnect a non-working cylinder, the sound will not change).
2. Turn off the engine and swap spark plugs from a “problematic” and a known good cylinder.
3. Start the engine again.

Results:

• 🔄 If "triple" has moved on the other cylinder - the spark plug is to blame.
• 🔧 If the problem remains on the same cylinder, look for the cause in reel, wire or nozzle.

This method also works for individual coils: just swap them. If the fault “moves” along with the coil, it must be replaced.

⚠️ Attention: On engines with direct injection (for example, GDI or FSI) "triple" may be caused by dirty injectors. In this case, rearranging the spark plugs will not help - the fuel system needs to be flushed.

Method 5: Bench Test - Professional Diagnostics

The most accurate, but also the most expensive method is checking spark plugs on a stand in service station conditions. Modern stands (for example, Bosch FSA 740 or Hella Gutmann) simulate real engine operating conditions:

• 🔥 Testing breakdown voltage at different pressures (up to 20 bar).
• 📊 Under construction discharge chart, showing spark stability.
• 🔍 Revealed hidden defects (microcracks, internal breakdowns).

The cost of checking one spark plug is from 200 to 500 rubles, but it is justified if:

• Do you suspect hidden defect (for example, a spark plug works at idle, but “disappears” under load).
• You have sports or boosted engine, where the ignition requirements are higher.
• you buy used car and want to check your service history.

You can also check at the stand ignition coils and high voltage wires, which will save time searching for the causes of the malfunction.

If you decide to buy a stand for personal use (relevant for car services), pay attention to models with the function dynamic testing — they simulate changes in engine speed.

When to change spark plugs: manufacturers' recommendations and real experience

Manufacturers of spark plugs and cars give different recommendations on service life:

Candle type	Recommended resource (km)	Real life (with high-quality fuel)	Examples of models
Regular (copper/nickel)	20 000 — 30 000	15 000 — 25 000	NGK BPR6ES, Bosch WR7DC
Platinum	60 000 — 80 000	50 000 — 70 000	Denso PK20PR-P8, Champion OE201
Iridium	100 000 — 120 000	80 000 — 100 000	NGK IFR6A11, Bosch FR8LI33X
Silver	20 000 — 30 000	15 000 — 25 000	Beru 14FR-7DU

However, the actual service life depends on:

• 🛢️ Fuel quality — additives and impurities accelerate the formation of soot.
• 🔥 Operating mode — frequent short trips (especially in winter) reduce the life of candles.
• 💨 Engine states — oil burn or overheating kills spark plugs within a few thousand km.

When to change spark plugs ahead of time:

• After antifreeze ingress into the cylinders (for example, due to a broken cylinder head gasket).
• After long machine downtime (more than 6 months) - the candles may turn sour.
• When installation of gas equipment — gas requires “hotter” spark plugs (with a lower heat rating).

Don't wait for the spark plugs to fail! Preventive replacement (for example, once every 50,000 km for iridium spark plugs) is cheaper than repairing the engine due to detonation or misfire.

Common mistakes when checking and replacing spark plugs

Even experienced car owners sometimes make mistakes that lead to damage to spark plugs or engine. Here are the most common:

• 🔧 Re-upholstery of candles - leads to damage to the threads in the block head. Tightening torque usually 20-30 Nm (indicated in the repair manual).
• 🧹 Dirt getting into wells — before unscrewing the spark plugs, be sure to blow out the wells with compressed air.
• 🔥 Using the wrong candles — an inappropriate heat number leads to detonation or misfire.
• 🛑 Replacing only one spark plug - if one candle fails, the rest will soon follow. Replace as a set!
• 💧 Installation without lubrication - the spark plug thread can be slightly lubricated copper paste (for example, Liqui Moly Kupfer-Spray) to avoid sticking.

Particularly dangerous incorrect tightening:

• Undrawn candle - will lead to loss of compression and overheating.
• Overstretched candle — the insulator may burst or break the threads in the aluminum cylinder head (repairs will cost 10-15 thousand rubles).

If you are unsure of the tightening torque, use torque wrench. For most flat-fit (non-cone seal) spark plugs, the torque is 25 Nm.

What to do if the candle is stuck?

If the spark plug does not come out, do not force it! Treat the threads with a rust remover (such as WD-40 or Liqui Moly MoS2-Spray), let stand for 10-15 minutes, then carefully unscrew using an extension cord. As a last resort, use broken spark plug extractor.

FAQ: Answers to frequently asked questions

Is it possible to drive with a faulty spark plug?

Short term - yes, but this leads to:

• Increased fuel consumption by 10-30%.
• Unburnt gasoline entering the catalyst (risk of its melting).
• Overloading the remaining cylinders and accelerated engine wear.

On modern cars with EFI electronics can turn off the injector of an idle cylinder, but this does not solve the problem - the spark plug needs to be replaced.

How to check spark plugs on a diesel engine?

On diesels no spark plugs - used there glow plugs. They are checked:

• Multimeter (resistance should be 0.5-6 ohms).
• At the stand (checking the heating time).
• Visually - if the tip is melted or broken, the spark plug must be replaced.

Glow plugs usually last 60-100 thousand km, but may fail earlier due to corrosion or mechanical damage.

Which spark plugs are better - iridium or platinum?

Comparison:

Parameter	Iridium	Platinum
Resource	100,000 km	80,000 km
Price (for 1 piece)	800-1500 rub.	500-1200 rub.
Spark quality	Better (thin central electrode)	good
Suitable for	Highly boosted and turbocharged engines	Atmospheric engines, HBO

For most modern cars iridium spark plugs preferable - they give a more stable spark and last longer. Platinum ones are cheaper and better suited for cars with HBO (gas requires a “colder” spark plug).

What happens if you put candles with the wrong heat rating?

The heat number determines thermal regime candles:

• The spark plug is too hot (low heat rating) - overheats, causes detonation, and can lead to piston burnout.
• Too "cold" candle (high heat rating) - quickly becomes covered with soot and misses ignition.

Always follow the vehicle manufacturer's recommendations! The heat rating is indicated in the operating instructions (for example, for VAZ 2114 this is A17