Reading spark plugs: what the deposits tell you about the cylinder

A spark plug is the only component with one end living inside the combustion chamber that also unscrews easily. Over tens of thousands of miles it accumulates a report on everything that happened in there.

Reading that report is easy once you know two rules.

Rule one: compare plugs against each other

The main mistake is looking at one plug and drawing a conclusion about the engine.

The value lies not in the colour itself but in the difference between cylinders. Four identical plugs mean a normal engine simply operating in that regime. Three identical and one different means a specific cylinder with a specific problem.

So remove all of them and lay them out in order, keeping track of which came from where.

Rule two: fresh deposits do not read

A plug after a short urban trip looks different from one after a highway run. For an honest picture you need 20–30 minutes of warm driving before removal.

What each picture means

Light brown, greyish-brown

Normal. That is how a plug looks on an engine where the mixture is correct and the plug’s heat range is right.

This is the target.

Black and dry, sooty

Rich mixture. More fuel than needed, and it does not fully burn.

What to cross-check: long-term fuel trim negative, code P0172. Causes range from a lying temperature sensor to a clogged air filter; covered in the rich mixture guide.

If only one plug is black — that cylinder’s injector.

Whitish, with electrode erosion

Lean mixture or overheating. The plug is running hotter than designed.

What to cross-check: trim positive, code P0171; covered in the lean mixture guide.

Also in this category: detonation and a wrong heat range. Detonation mechanics are in the knock sensor guide.

This is the most alarming picture of all: lean mixture plus overheating destroys pistons.

Oily, glossy deposits

Oil is reaching the chamber. Valve stem seals, piston rings, or on a turbo engine the turbocharger seals.

What to cross-check: oil consumption on the dipstick, P0420 from a damaged catalyst, misfires in that cylinder.

One plug of four points at a specific cylinder. All four means a general problem.

Wet, smelling of fuel

Fuel is not burning in that cylinder at all. No spark, or the injector is leaking.

What to cross-check: misfire counters, codes P0301–P0304, injector circuit codes P0201–P0206; covered in their own article.

Reddish or yellowish coating

Fuel additives. Not a fault, but a signal to look at fuel quality — see the bad fuel guide.

Melted electrode, cracked ceramic

Severe detonation or pre-ignition. The most serious picture available: pistons are next.

Causes: low-octane fuel, chamber carbon, overheating, too “hot” a plug, an aggressive map — see detecting an ECU remap.

What else to look at beyond colour

The gap. It grows with mileage through electrode erosion. A wider gap demands higher voltage and the coil works at its limit — hence misfires under load on a plug that looks entirely normal.

Thread and sealing washer condition. Gas blow-by marks along the thread mean the plug was under-tightened.

Moisture in the well. Unrelated to combustion, but it explains misfires — typical after an engine bay wash; covered in the washing article.

The procedure

1. Drive 20–30 minutes before checking.
2. Read codes and misfire counters before removing the plugs. The data and the parts should agree.
3. Remove all plugs and lay them out by cylinder number.
4. Compare them against each other — the difference matters more than absolute appearance.
5. Cross-check against fuel trims.
6. Measure the gap even on plugs that look fine.

When to replace

Service life depends on type: conventional 12,000–20,000 miles, platinum and iridium 40,000–60,000. The figures are indicative; the manufacturer’s schedule takes precedence.

Important: a plug that looks fine but has exhausted its service life is still a replacement candidate. A worn electrode produces misfires under load that are invisible at idle.

Plugs and coils together are covered in their own article.

In short

• Read the difference between cylinders, not the colour of one plug.
• Drive 20–30 minutes warm before checking, or the picture is unreliable.
• Black and dry means rich; whitish with erosion means lean or overheating.
• Oily deposits on one plug point at a specific cylinder.
• A melted electrode means detonation, and pistons come next.

Download the app — fuel trims and misfire counters are read with an ELM327 adapter over Bluetooth SPP on Android; cross-check them against what the plugs show.