The #7 Toyota GR010 Hybrid, which had started the lap in the lead of the race, ended up on its roof after Lopez went off at Bishop Bend after hitting the barriers earlier on the lap at Turn 9 following contact with Julien Andlauer’s Dempsey Proton Porsche 911 RSR.
Lopez appeared to suffer some kind of failure as he made his way back to the pits with smoke coming from the left-front corner and failed to negotiate the left kink at Turn 15.
The Argentinian driver was able to extricate himself from the car unaided and was taken to the circuit medical centre.
Toyota reported that he had suffered no injuries.
The clock remained running with the field lined up on the backstraight opposite the WEC pits, with a likely restart time of 4:05pm local time.
Lopez had just taken the Toyota he shares with Kamui Kobayashi and Mike Conway into the lead of the race from the sister GR010 driven by Sebastien Buemi, Brendon Hartley and Ryo Hirakawa.
Hartley sat at the top of the classification when the red flag was shown, with a lead of the 19s over the Alpine A480-Gibson grandfathered LMP1 with Andre Negrao at the wheel.
Both Toyotas were due for pitstops and would have fallen behind the Alpine that had dominated the first three hours of the race.
The Signatech-run Alpine had a lead of over a minute when Matthieu Vaxiviere pitted 15 minutes before the stoppage to hand over to Negrao.
The Oreca-designed car had a clear advantage in the Hypercar class around the Sebring International Raceway over the first half of a race that can run for a maximum of eight hours.
Vaxiviere was able to pull away from the rest of the field at a rate of a second a lap after the midday start.
Prema led LMP2 on its WEC debut with the Oreca 07-Gibson shared by Robert Kubica, Louis Deletraz and Lorenzo Colombo.
Deletraz moved into the lead ahead of the two United Autosports entries at the fifth round of pitstops early in the fourth hour when he stopped for fuel and didn’t take a new set of Goodyear tyres.
The Chevrolet Corvette C8.R headed the GTE Pro field with Tommy Milner at the wheel.
Milner held a 17s lead over Kevin Estre in the best of the two factory Porsche 911 RSR-19s, which were running second and third when the red flag came out.
Both Porsches were held for 15s at their second pitstops for infringements during the rolling start procedure.
GTE Am was led by the NorthWest AMR Aston Martin shared by Paul Dalla Lana, David Pittard and Nicki Thiim.