Resistor (RFI-suppressed) spark plug
That little letter R buried in a code like BR6ES 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.
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: BR6ES 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. W20EPR-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.
| Situation | Result |
|---|---|
| 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.
A common slip: an owner finds a B6ES on the shelf and fits it instead of the specified BR6ES 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 (BR6ES 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.