This is why brown and yellow deposits form on spark plugs

Spark plugs are essential to an internal combustion engine, especially for the ignition process. The spark plugs also have various problems, some of the symptoms are the colors on them that tell specific problems related to them, but these colors may indicate a possible engine problem.

The colors can vary being black, white, red, brown, and yellow, each of these colored deposits mean something.

Brown and yellow buildups can form on spark plugs, specifically on the insulator. We will cover the possible causes of why these colored deposits form in this article.

In previous articles, I have covered why black and white deposits form on spark plugs. You can check those articles out.

It is possible to identify why these brown and yellow deposits form on the plug’s igniter only through correct diagnostics. A visual inspection rarely helps to solve the problem, but sometimes car owners can easily cope with this task.

What are the causes of brow deposits on spark plugs?

Brown deposits form on spark plugs after prolonged use. This color does not only look very weird, but it can also mean that there is a problem.

Low-quality fuel

Usually, a common cause of why brown deposits on spark plugs form, is the use of low-quality fuel, most experts in the automotive field agree with this.

Fuel issues cannot be determined with the naked eye by looking at the spark plug insulator after a while. The brown spots may differ in shade and structure, thus only after a correct examination of the plug, it will be possible to identify the exact reasons for this problem.

Due to a poor operation of the injector, which has occurred after exposure to blockages from low-quality fuel, the fuel begins to fill the spark plug.

As a result, a brown deposit will appear on the spark plug insulator, and it will look rusty, the electrode will not be able to burn out the excessive amount of the supplied mixture, and part of it is soaked through the metal body of the igniter.

Other causes

A rusty brownish insulator can have different shades of this color. Based on this, you can accurately determine the faulty part of the car. A darker shade indicates that the fuel mixture entered into the combustion chamber due to a clogged air filter.

If the brown is more reddish, this can mean problems with the piston rings that need to be replaced, in which case the oily liquid enters the combustion chamber, leaving marks on the insulator over time.

The unsuitability of the caps for connecting the mass to the igniters is not excluded; it is necessary to periodically replace these components.

A reddish-brown color is a sign of gasoline additives with added iron and carbon. All shades of red, no matter what tint or other colors it mixes with, are associated with poor fuel and the presence of iron-containing subsidence in it. 

Why do yellow deposits form on spark plugs?

Another issue with the spark plugs represents the yellow deposits on them. The most common cause why this happens is also the use of low-quality fuel that has additives which may lead to the formation of these yellowish deposits.

In the mixture, there is an increased presence of bad additives that the fuel has in it. In the short term, it does not pose a serious threat, but in the long run, it can damage the engine entirely.

If you see a yellowish buildup on a spark plug, this means that gases leak from the cylinder. Because of this, the insulator turns yellow, and a dark spot appears between the metal part and the ceramic.

These yellow deposits will tell you that the spark plugs are bad and need to be replaced. A sign that spark plugs have these yellowish buildups is that drivers face problems when starting the engine.

Other reasons for yellow deposits on spark plugs can be:

  • Problems with some individual car parts;
  • Poor engine efficiency.

If you see yellow spots on the spar plug’s electrode, you should check the correct operation of the valves or partitions in the cylinder, they may have worn out.

Often, such manifestations are accompanied by drops of oil on the electrode and a small number of metal crumbs. The symptom is that the engine will lose some of its power shortly after starting.

On ceramics In addition to changing to a higher quality fuel, you need to think about the wear of the caps for replenishing the igniters.

These parts become excessively hard over time and cannot adhere so well to the ceramic body without stopping to discharge when the engine is started.

The yellow color on spark plugs may appear due to a discharge. If that is the case then it is better to replace the armored wires of the engine. You can see the discharge by opening the hood and starting the engine in the dark, the blue glow around the spark plugs will give away.

Like in the case of brown deposits, a visual inspection does not help to solve the problem, only a correct diagnose at an authorized repair shop.

If you see any bad signs in the engine’s proper functioning, especially bad or hard start you should check the problem thoroughly.


Often these deposits on spark plugs indicate a problem with these parts, but can also indicate problems with other engine parts. Because it is hard to diagnose, you should check this issue thoroughly at an authorized repair shop to find the culprit.

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