Truck lighting violations can seem puzzling. A problem bulb allowed at one roadside inspection station might place a vehicle out-of-service further down the highway.

Kerri Wirachowsky – the Commercial Vehicle Safety Alliance’s director, roadside inspection program – says there’s likely a good reason for the difference.

How can one enforcement officer allow me to continue a trip with a burnt marker light, while another puts me out of service?

The first thing with lighting is you’re only out-of-service when the light’s required.

If I stopped you in the middle of the afternoon and I see that you have no taillights, it will be a violation — but you will not be placed out-of-service. Seven hours from now, if somebody stops the same truck with the same violation, they’re going to be placed out-of-service for no taillights.

What about LED lamps? How many bulbs are allowed to go out before it’s a violation?

There is no violation standard for LED lights as a whole.

If I’m standing on the side of the road, and I look at an LED-type light, there’s diodes in there. Depending on the LED, depending on the size of the diodes, they go out in clusters. How many have to be out before the vehicle is in violation? There is no number.

That light is required to be seen from 500 feet. Inspectors have to make that call as to whether that light, whatever remaining diodes are there, can be seen from that distance.

With some of the smaller box-type trailers, the ID lights and clearance lights consist of one little LED. One will cause it to be inoperative and I can’t see it.

How should drivers document LED light failures?

If your drivers document every time they see some diodes out, you’ve got an opportunity to fix that problem before it even becomes a violation. They aren’t probably all going to go out at the same time unless you’ve got a wiring issue.

You get a chance to fix that problem before anybody’s going to cite you for a violation.

Why do I receive a lighting violation if the only problem is an unplugged pigtail? There’s no mechanical issue.

When you have a pigtail that’s out, all the lights on the back of the trailer don’t work. You’ve got clearance lights not working, ID lights not working, brake lights, tail lights, turn signals, licence plate lights, all of them.

If I stop a vehicle with no lights on the rear, they are going to be placed out-of-service.

From an enforcement standpoint, if I stop your tractor-trailer, whether it takes you five seconds or five hours to fix the violation, the violation is still there. If the repair is the driver coming out and plugging in the pigtail, so be it. But if I didn’t stop you, he’d have been driving down the road for who knows how long with no lights to the rear.

When the driver plugs in the pigtail, I check the lights again. If the lights are all functioning, it’s one violation.

If some of those other lights are still not functioning, now the inspector will add the applicable lights that are actually inoperative to his inspection report, as well as the violation for the pigtail not being plugged in in the first place.

Why are some fleets reporting more violations for ABS lamps?

When your ABS stops functioning, your foundation brakes don’t break. They still work. However, it is a violation in the regulations if your ABS is not functioning, or if your ABS malfunction lamp comes on and stays on, or doesn’t come on at all.

Enforcement people are not trained to test ABS. But they are trained to check the ABS malfunction lamp on both the tractor if equipped, and the trailer if equipped.

Most drivers and most inspectors initially thought, ‘I just turn a key, ABS malfunction lamp comes on and goes off, and all as well.’ That happens sometimes. But there are other times where you will not be able to cycle the trailer ABS malfunction lamp unless you either put your foot on the brake, or unplug the pigtail and plug it back in.

We have focused on it and tried to train inspectors on how to do it. And when we focus on something and train them on it, then they tend to go out there and look for it.

• This conversation has been edited for length and clarity.