How to determine which ignition coil is not working: symptoms and test methods

Why is it important to identify a faulty ignition coil in time?

The ignition coil is the heart of the fuel mixture ignition system in gasoline engines. When it fails, even one inoperative cylinder can lead to loss of power by 25-30%, an increase in fuel consumption by 10-15% and the risk of damage to the catalytic converter. At the same time, modern cars with individual coils (one for each cylinder) allow you to accurately localize the problem - if you know how to do it.

Many drivers mistakenly attribute the symptoms of a faulty coil to “bad gasoline” or “electronic glitches.” Meanwhile, ignoring the problem is fraught chain reaction: misfire → overheating of the catalyst → its melting → entry of ceramic dust into the cylinders and scuffing on the walls. In this article, we will look at how to independently identify a non-working coil without expensive diagnostics - from the simplest methods to professional techniques.

Top 5 symptoms of a faulty ignition coil

The first signs of a problem often appear before the error appears. P030X (where X is the cylinder number) on the dashboard. Experienced mechanics identify five key symptoms that should alert you:

• 🔥 Engine tripping - the most obvious sign. The engine “tweaks” at idle or under load, vibrations are transmitted into the cabin. On V-shaped engines (for example, Toyota 1GR-FE) tripling may be less noticeable due to design features.
• ⚡ "Disruptions" during acceleration — the car jerks when accelerating, especially in the range of 2000-3000 rpm. More common on cars with turbocharged (Audi 1.8T, BMW N20).
• 🛢️ Increased fuel consumption — when the cylinder is not working, the ECU compensates for the loss of power by enriching the mixture. For example, on Kia Rio 1.4 consumption can increase from 7 to 9-10 l/100 km.
• 🔧 Check Engine Error - codes P0301-P0308 (misfire in cylinders 1-8, respectively) or P0351-P0358 (coil circuit malfunction). On some models (Ford Focus 2) only the “engine” icon may light up, without a specific code.
• 🌡️ Catalyst overheating — unburned fuel burns out in the outlet, which leads to the catalyst body heating up to 900-1000°C. You may hear a metallic ringing sound or smell sulfur.

Important: on vehicles with cylinder deactivation system (for example, Honda VCM or GM Active Fuel Management) tripping can be a sign not only of a faulty coil, but also of problems with valves or hydraulic compensators. In this case, a comprehensive diagnosis is required.

Method 1: Visual inspection - what can the appearance of the coil tell you?

Before using tools, inspect the coils for external defects. This method does not require special skills, but allows you to identify up to 30% of faults. What to look for:

• 🔌 Cracks on the body - especially in places where high-voltage wires are attached (on old Bosch coils) or power connector. Through them, moisture gets inside, causing short circuits.
• 💧 Traces of oil or antifreeze - indicate a leak through the valve cover gasket (typical for Renault K4M or Peugeot EP6). The liquid destroys the insulation of the windings.
• 🔥 Charring or melting - a sign of overheating due to an interturn short circuit. Often found on reels Delphi after 100,000 km.
• 🧲 Rust on contacts — oxidation of connectors leads to loss of signal from the ECU. This is especially true for vehicles operated in regions with salty roads (for example, Volvo S40 in coastal cities).

⚠️ Attention: If there is a white coating on the coil (especially at the base), this is a sign corona discharge — high voltage “breaks through” the air to ground. In this case, the coil must be replaced, even if it is still working. Ignoring the problem will damage the ECU.

For inspection, remove the coils (on most engines it is enough to disconnect the connector and unscrew 1-2 bolts). On motors with aluminum head (for example, Mitsubishi 4G63) be careful - the threads in the cylinder head often “lick off”.

Disconnect the negative battery terminal

Take a photo of the location of the connectors (on some cars they are different!)

Check the integrity of the rubber cap (on reels NGK it often cracks)

Inspect the spark plug well for moisture or carbon deposits-->

Method 2: Checking for a spark - myths and reality

Many “garage mechanics” advise checking the coils by unscrewing the spark plug and looking for a spark. This method extremely dangerous for modern cars for three reasons:

1. Risk of damage to the ECU - the voltage in the ignition system reaches 40,000 V, and if the spark “shoots” in the wrong direction, the transistor switch in the control unit may burn out.
2. False results - a weak spark in air does not mean a malfunction. The conditions in the cylinder are different (pressure 10-12 atm, temperature 500°C).
3. Injury hazard - electric shocks of 20,000 V are not fatal, but are extremely painful and can cause reflexive movement with consequences.

If you do decide to check the spark, do so. only with the help of a spark gap (for example, Bosch FSA 740) or through spare spark plug, securely attached to the ground. Algorithm:

1. Disconnect the fuel pump (remove fuse or relay).
2. Unscrew the spark plug from the “suspicious” cylinder and insert it into the coil cap.
3. Press the spark plug skirt firmly against a metal part of the engine (such as a valve cover).
4. Crank the starter for 2-3 seconds. There must be a spark bright blue, without interruption.

⚠️ Attention: On engines with direct injection (for example, VW TSI or Mazda Skyactiv-G) this method is not informative, since fuel is supplied directly to the cylinder, and without it the spark will be weak even on a working coil.

Method 3: Diagnostics with a multimeter - step-by-step instructions

Testing with a tester is the most reliable way to identify a coil malfunction. You will need a multimeter with a resistance measurement mode (up to 200 MOhm) and exact data for your coil model (they can be found in the manual or on the manufacturer’s website, for example, Denso or Beru).

The coil has two windings:

• Primary - low voltage (0.5-2 Ohm), responsible for creating a magnetic field.
• Secondary - high voltage (6-15 kOhm), generates a spark.

Verification algorithm:

1. Primary winding:
   • Connect the probes to the contacts 1 and 3 (at the coil connector).
   • Resistance must be within 0.5-2 Ohm (for Bosch 0 221 504 015 — 0.7±0.1 Ohm).
   • Indications 0 ohm - short circuit, ∞ - break.

Working coil:
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_| |_ (sharp rise, flat top)

Faulty (turn-to-turn short circuit):
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_ / \_ (flat front, “smeared” top)