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Ask a Mechanic......DRL/lighting followup


My Dec. 24 article about daytime running lights and automatic light activation generated several related responses from readers. Here are two of them.


Dear Ask a Mechanic,

I normally have our light switch set to automatic but have learned from experience to check the setting immediately after having the car serviced.  I have found that the service people do not make a mental note of the switch setting before checking all of the lights and so by default just turn it to off when they are finished.  The first time this happened, because the (DRLs) were on, we did not realize there was a problem until another motorist informed us. – Gary Fletcher – Stouffville

Gary, you’re right that techs should remember to return settings to where they were. I’ll admit to occasionally forgetting it myself.

The probable reason the headlight switch is changed in the first place has to do with two things.

The first is that it’s nearly always dark enough inside repair facilities that the automatic lights activate.

The second is that the two most typical situations when performing services tend to leave auto lamps systems on, quickly killing the battery:

  1. The vehicle is not in park and/or the steering lock is deactivated, since many services require techs to be able to move the steering and/or rotate the wheels.
  2. “Ignition on, engine off” as needed for diagnostics/programming or resetting reminders.

To avoid that battery drain, the auto lamps must be switched off.

When I first noticed this, I thought there was an unusually high number of cars that had the “one-eyed” headlight syndrome. Approaching an intersection, a vehicle with its turn signal on had no light on the same side they were signaling. After a while, I figured out it was an intentional feature that the manufacturer had incorporated. I think it’s confusing and at times even somewhat dangerous. Can you explain why this has become a standard feature on many makes and models? – Ken Harker – Tyrone

Ken, you may find it ironic that the entire reason that the DRL (daytime running light) turns off on the side that a signal is flashing is actually to improve safety; it’s written into Canada’s motor vehicle standards. There’s a bunch of “if/thens” involved, but the gist of this requirement is that if the DRL might obscure the light of the signal due to brightness and/or proximity, it must turn off to make the signal more visible.

Ask a Mechanic is written by Brian Early, a Red Seal-certified automotive technician. You can send your questions to These answers are for informational purposes only. Please consult a certified mechanic before having any work done to your vehicle.


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