Anglian Water says it is working on Thorney Hill, near Peterborough, to repair the pipe.
Parish councillor Ruth Horton is critical of a revised route for the previously stalled bypass.
The boy was struck on the A47 by a car and is in hospital with life-threatening injuries, police say.
The air ambulance is attending the incident near Wisbech.
A driver is held on suspicion of causing death by dangerous driving and death by careless driving.
A stretch of the road is fully closed after the crash but both carriageways are now open.
Gas engineers say a new issue has been found at a mains pipe just three months after earlier repairs.
Orsted says it is taking place on Sundays to reduce the impact on traffic.
Police are appealing for information after the collision involving two cars and a lorry on 21 May.
