# Resistor (RFI-suppressed) spark plug

> A resistor plug contains a resistor that suppresses electrical interference.

That little letter **R** buried in a code like B**R**6ES isn't decoration — it tells you the plug has a **built-in resistor**. On a modern engine it's not optional, and the most popular myth about it ("pull the resistor out for a fatter spark") is exactly backwards. Here's what the resistor really does and why it matters.

## What a resistor plug actually is

Every spark is a tiny, violent electrical discharge. As the current jumps the gap it doesn't just light the mixture — it radiates a burst of **electromagnetic and radio-frequency interference (RFI)**, the electrical equivalent of static. A **resistor spark plug** has a small resistive element (typically around **5 kΩ**) built into the centre electrode, between the terminal and the firing tip. That resistor damps the sharp current spike and **suppresses the interference** the spark would otherwise broadcast.

Hence the other common name: an **RFI-suppressed** plug. Same spark, same combustion — just without the electrical noise leaking out into everything around it.

## Why the interference matters

In the old days the only victim of ignition noise was the AM radio: that rhythmic crackle that rose and fell with engine speed. Annoying, but harmless. On a modern vehicle the stakes are completely different, because the engine is surrounded by **sensitive electronics**:

- the **ECU** (engine control unit) and its low-voltage logic;
- dozens of **sensors** reading tiny voltages — crank, cam, knock, oxygen;
- the **CAN bus** and other data lines carrying digital signals around the car.

RFI from an unsuppressed spark can couple into those circuits, corrupt a reading, and make the engine **run erratically** or trip **fault codes** — most commonly phantom **misfire** codes where nothing is actually wrong with combustion. You end up chasing a ghost.

Good to know

The resistor also protects the ignition itself. By smoothing the discharge it reduces electrical erosion and stress on the **coil and ignition module**, and helps the electrodes last. So it's not only about the radio and the ECU — it's gentle on the hardware that makes the spark in the first place.

## How to spot one: the letter R

You don't need a meter to read the code. Manufacturers flag the resistor with an **R** in the part number:

- **NGK**: B**R**6ES is the resistor version of B6ES. Same plug, same heat range — the R is the only difference.
- **Denso**: the resistor types carry a **U** in the code (e.g. W20**EPR**-U style markings).

If you want to confirm it physically, set a **multimeter** to resistance (ohms) and measure from the top terminal down to the centre electrode. A resistor plug reads roughly **4–6 kΩ**; a true non-resistor plug reads close to **0 Ω** (a dead short).

## Compatibility: match like for like

The safe rule is simple: **fit what the engine was specified with.** But the two mistakes are not equally serious.

SituationResult
**Non-resistor** fitted where a **resistor** is required (any modern, ECU-managed engine)**Risky.** Interference, radio static, possible fault codes, erratic running.
**Resistor** fitted where a **non-resistor** was specified (old engines, magneto ignition, some race setups)**Usually harmless.** The extra ~5 kΩ is negligible; the engine simply runs clean.

In other words, when in doubt, a resistor plug is the conservative choice. The only places to be careful are **magneto** systems and dedicated **race** applications that were deliberately built around non-resistor plugs — and even there, the manufacturer's recommendation is the final word.

## The "remove the resistance for a stronger spark" myth

This is the one to bury. The idea that deleting the resistor — or fitting a non-resistor plug on purpose — gives you a hotter spark and more power is **simply false**. The 5 kΩ resistor does **not** rob the spark of meaningful energy; combustion is identical. What you actually "gain" by removing it is:

- **no** measurable power increase;
- a flood of **RFI** into your electronics;
- a real chance of **misfire codes** and rough running;
- extra electrical stress on the **coil and module**.

Concrete example

A common slip: an owner finds a B6ES on the shelf and fits it instead of the specified B**R**6ES because "it's the same plug, cheaper." A week later the dash lights up with a misfire code, the engine hunts at idle, and the Bluetooth keeps dropping. Nothing is mechanically wrong — the missing **R** is broadcasting interference straight into the ECU. Swapping back to the resistor plug clears it instantly.

## In short

- A resistor plug has a built-in resistor (~**5 kΩ**) that suppresses the spark's **RFI**.
- That interference disturbs the radio and, far more importantly, the **ECU, sensors and CAN bus** — it can trigger misfire codes and erratic running.
- Spot it by the **R** in the code (B**R**6ES vs B6ES); confirm with a multimeter (~4–6 kΩ vs ~0 Ω).
- Modern, electronically managed engines **require** a resistor plug. Match like for like.
- Wrong way (no resistor where one is needed) = interference and fault codes. Other way (resistor on an old/race/magneto engine) = usually harmless.
- "Remove the resistance for a stronger spark" is a **myth** — no power gain, just noise and risk to your electronics.

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*Source : [Resistor (RFI-suppressed) spark plug](https://www.thesparktwin.com/en/guide/resistive/) — TheSparkTwin*