A key test to measure pressure inside BP's ruptured Gulf of Mexico well is under way, the company said Thursday.

The test will take from six to 48 hours, and successful results could lead the way for the flow of oil to finally be stopped.

Earlier Thursday, BP replaced a leaking piece of equipment in preparation for the pressure test, which is key to determining whether a new cap can finally seal off oil that has been gushing into the Gulf of Mexico for the last 12 weeks, said Senior Vice President Kent Wells.

BP plans to close off -- one by one -- the valves through which oil can escape, said retired Adm. Thad Allen, the government's response manager. It was in the process of doing that Wednesday when a leak was discovered in the choke line, forcing BP to abandon operations until the line was replaced earlier Thursday



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Scientists and engineers will monitor the pressure every six hours and evaluate the situation with the new undersea containment cap put into place earlier this week.

If the pressure readings are satisfactory, the valves on the custom-made stacking containment cap could remain closed, signaling the beginning of the end to the catastrophe that began when the Deepwater Horizon drilling rig exploded April 20, killing 11 workers and triggering the relentless oil spill.

But if the pressure readings are low, BP will know that oil is seeping out through another part of the well as well, sort of like a leaky garden hose that dribbles out water when you've got your thumb on the nozzle, Allen said.

The cap was not designed to permanently shut in the well -- it was meant to move to a four-vessel containment system and assure redundancy in the event of a hurricane. But Allen said there could be a huge side benefit if the oil can be contained -- a "twofer," as he called it.

Allen said the more permanent solution to the spewing oil remains the two relief wells BP is drilling and expects to have them finished in August.

BP pumped drilling mud into those relief wells to mitigate risks during the pressure testing. The two wells intersect with the Macondo.

Oil recovery was stopped Wednesday ahead of the integrity test but resumed while BP was fixing the problem with the leaking choke line. It will stop again as the testing nears.

Wells said BP collected 537,600 gallons of oil Wednesday. Government scientists estimate between 1.5 million to 2.5 million gallons are flowing into the Gulf every day.

A key question over the pressure tests was whether shutting the well was worth the risk, or whether they might cause fresh damage to the blowout preventer.