Can you replace just one garage door spring?
On a door with two torsion springs, no, and the reason is not upselling. The two springs went on the same day, have cycled the same number of times, and are built to wear out together. When one lets go, the other has weeks left. Replacing only the broken one leaves the door pulling unevenly, which strains the opener and the cables, and it brings the pro back for a second service call within the season. A pair goes on together, $260 to $460 installed on a Dunwoody two-car door, and the door is balanced as a unit.
The case where one spring is right
A single 9 by 7 door on the older ranches often has one torsion spring over the header, and a one-spring door gets one spring: $180 to $300. The 1970s doors with extension springs stretched along each track are a pair by design, and they are replaced as a pair for the same reason the torsion pairs are.
Why both springs break within weeks of each other
A torsion spring is rated in cycles, one cycle being the door going up and coming down. A standard spring is rated around 10,000 cycles. A two-car family that uses the garage as the front door runs four to six cycles a day, which puts a standard pair at seven to ten years. Both springs on a door count the same cycles from the same day. When the first goes, the second is at the same number.
What happens when only one is replaced
The new spring is at full strength and the old one is nearly spent, so the door lifts unevenly. The opener carries the difference, which is what strips its drive gear. The cable on the weak side sags and can jump the drum, which is how a spring job turns into an off-track call. And the old spring breaks anyway, usually within the season.
High-cycle springs
If the door is the front door of the house, a high-cycle pair rated around 20,000 cycles costs more up front and roughly doubles the years between spring jobs. On a 16-foot double that is used a dozen times a day it is the better buy. On a detached garage that opens twice a week, a standard pair is fine.
What to tell the pro
Whether it is a single or a double door, whether there is one spring over the header or two, or springs along the tracks, and whether the door has been repaired before. That is enough for a real price on the phone, and the visit runs about an hour.
When it is time to call: broken spring repair in Dunwoody, about an hour on site.