Simple Plug Checker: Go/No-Go on Resistance

Although it’s a bit pricey at $130, this gadget makes quick work of checking spark plugs for high resistance values.

The list of approved maintenance items an aircraft owner can do under FAR Part 43 is long and includes replacing, cleaning and gapping plugs. In the spirit of cleaning, we think checking plug resistance is also on the list.

Like lowering your cholesterol and cutting back on salt intake, plug resistance wasn’t always a concern, or at least one that anyone bothered with. But with the widespread use of engine monitors that can detect the slightest combustion anomalies, spark plug resistance has become the latest fine-point maintenance item owners need to worry about.

What’s the problem here? As we reported in the November 2012 issue of Aviation Consumer, aviation spark plugs have a built-in resistor to keep the spark from becoming too energetic, which will prematurely erode the electrodes. However, if the resistance is too high, excessive lead fouling, inconsistent plug firing and high CHTs may result.

But what’s too high? The nominal resistance of a new plug is 1000 to 2000 ohms, but as a plug ages, that resistance may double or triple or even more. One maintenance shop we know told us it found resistance as high as 15,000 to over a million ohms in some Champion plugs removed from engines and found to be heavily lead fouled.

Although Champion disputes reports of high resistance in its plugs causing problems, its arch competitor, Tempest, argues otherwise. And Tempest believes that all spark plugs should be resistance-checked during routine maintenance, so they’ve developed a device to do this.

As shown in the photo below, it’s nothing but a simple resistance bridge gadget with a go/no-go test protocol. It’s called the AT5K, which means at 5000 ohms or more, the spark plug should be replaced.
We tried the AT5K on a half dozen plugs, all of which tested within limits. Using the device is simple. Just mount the plug on a probe, which connects the harness end of the plug, then touch the tester’s lead to the center electrode. If the indicator glows green, the plug is good; if it’s red and green, the plug is serviceable. A red light indicates the plug’s resistance is greater than 5000 ohms and it should be replaced, at least according to Tempest.

A couple of caveats. The plug should be cleaned before being tested, otherwise it’s difficult to get the lead to make clean contact. Second, the plug should still be bomb tested to confirm a clean, robust spark.
At $129.75 from Aircraft Spruce, the AT5K is a bit pricey for what it does, in our view. If you have a good multimeter, you can test plugs individually. But with some meters, the probe won’t reach to the bottom of plug’s center well and even it will, finding a good contact is awkward. If you’ve got a six-cylinder engine and check plugs frequently, the AT5K may be worth the investment. For more information, see or aircraft

Paul Bertorelli is Aviation Consumer’s Editor at Large. In addition to his valued contributions to Aviation Consumer, his in-depth video productions on sister publication AVweb cover a wide variety of topics that greatly contribute to safety, operation and aircraft ownership. When Paul isn’t writing or filming, he’s out flying his J3 Cub.