I'd like to find a way to test my glow plug relay before dropping $135+ on a new one. Assume it's original, with 140K miles and most of its life spent in Oklahoma. (It's now in Virginia.)

When I turn the key "on," the GP lamp illuminates, then goes out after a suitable interval. Engine starts and runs fine. Same whether hot or cold. Same whether or not I wait a few extra seconds before starting the engine.

But the GP lamp occasionally lights while driving, too. It always goes off within about 60 seconds (usually much less), but it shouldn't be coming on at all unless the engine is running rough, and it isn't when this happens. It's always at least 10-20 minutes into a drive.

When the GP lamp comes on while driving, it turns the check engine light on. I can reset the CEL with an OBD tester, but it comes on again, and stays on, the very next time the GP lamp comes on while driving. I can repeat this over and over.

With the CEL on, the OBD tester reads code P0380, "glow plug/heater CKT A malfunction."

I don't think it's the glow plugs. They're brand new, Bosch, and these symptoms were present before I replaced them. Resistance tested through the wiring harness from the relay connector shows 0.5-0.6 ohms for GPs 1-4 and 6, 0.9-1.0 ohms for GP 5. I haven't tested actual current draw because my multimeter (Fluke 8022B digital) can't handle more than 2.0 amps DC.

Based on other posts, I pulled the relay and inspected all the solder joints. No breaks found.

Is there a way to test the relay?