Preparations are behind schedule for a critical firing of space shuttle Discovery's three main engines on the launch pad, and next Thursday's test might have to be postponed a day or two, NASA reports.

Shuttle managers planned to discuss the schedule Friday.The 20-second firing is considered critical to certify Discovery for the first shuttle launch since the Challenger exploded Jan. 28, 1986. It will test modifications made in the turbopumps and other engine parts and provide valuable countdown experience for the launch team.

But officials said Thursday that a hazardous gas detection system was not thoroughly checked out and there are questions about a quick disconnect device on an oxidizer line and a new connector pin on an electrical system.

A postponement would be yet another setback for the Discovery mission, already delayed five times since a launch date was set for last February. Liftoff is now scheduled for early Setember.

The main engines are separate from the steering engine system, which contains a small leak that engineers are also puzzling over, hoping to fix it on the pad without having to take Discovery back to a hangar for repair.

Removing the shuttle could delay the launch up to two months.

Experts have several innovative suggestions for fixing the leak of nitrogen tetroxide gas, which normally combines with fuel to power the steering engines.

The leak is six feet from the nearest entry panel and is blocked from normal access by a maze of tubes, wiring and tanks.

Taking a cue from heart doctors, engineers are considering running a thin tube with an expandable balloon on the end through a 20-foot line with six sharp bends in it. It would be something like a medical procedure known as an angioplasty, but instead of opening a clogged artery, it would close off the leaking line.

The materials and processing lab at the Marshall Space Flight Center in Huntsville, Ala., is devising a mockup of the 20-foot line to test this approach.

Another idea being explored is to enter Discovery's cargo bay and cut holes through both the rear of the bay and the engine compartment to reach the leak, then plug it by some method, including encapsulating it with a clamshell-like device filled with a sealant.

Engineers hope to begin testing all suggested approaches by this weekend.