United Launch Alliance isn’t taking any chances with its new Vulcan Centaur rocket opting Thursday to stand down from a planned hot fire test and continuing the potential delays before its debut mission dubbed Certification-1.

Ahead of plans to light up the engines at Cape Canaveral Space Force Station, teams saw a delayed response from the booster engine ignition system that would power up its two BE-4 engines supplied by Blue Origin.

“Timing and response doesn’t look right. Need to understand it,” ULA President and CEO Tory Bruno posted in an update on Twitter.

The Flight Readiness Firing test at Canaveral’s Space Launch Complex 41 was going to be the first time the combined Vulcan first stage topped with the Centaur second stage would light up on the launch pad, although held down with restraints.

The six-second firing would also test out the company’s pre-launch timelines and procedures, propellant loading and a target of six seconds of engine burn producing close to 1 million pounds of thrust.

“FRF is really about confirming the operational readiness of the integrated system: launch vehicle, ground systems, facilities and the associated software. In addition, we will demonstrate the ability to successfully execute the engine start sequence and validate our hot-fire abort response procedures,” said Dillon Rice, ULA’s Vulcan launch conductor in a press release.

Instead, the rocket will be rolled back to ULA’s Vertical Integration Facility so teams can get access to the booster ignition system. Depending on what they find, the rocket may roll back to the launch pad to proceed with the FRF ahead of its oft-delayed Certificaiton-1 mission that likely won’t fly until July at the earliest, Bruno indicated earlier this month.

Its debut mission is the first of two required before it can move forward with several Department of Defense flights. Vulcan Centaur is the replacement rocket for ULA’s remaining Atlas and Delta IV Heavy rockets.

Certification-1’s primary payload is the Astrobotic Peregrine lunar lander on what would be the first of NASA’s Commercial Lunar Payload Services (CLPS) missions to the moon. Vulcan is also flying up the first two of Amazon’s planned Project Kuiper constellation of satellites, as well as the ashes of more than 150 people including “Star Trek” creator Gene Roddenberry and actor James Doohan, who played “Scotty” on the TV series and movies.

Those payloads are not part of the FRF test, with Astrobotic’s lander not even in Florida yet.

The BE-4 engines have already been tested at Blue Origin’s facilities, but have yet to test fire while attached to the rocket. One of the engines already had to be removed and analyzed after issues with its power output were detected.

The test fire, though, isn’t the only issue that could cause further delays to the Certification-1 mission, which already missed a May 4 announced launch target this year, but also was expected to fly as early as 2021 before delays blamed on engine delivery from Blue Origin, COVID-19 and ULA’s own rocket manufacturing.

In fact, ULA has yet to complete testing of the Vulcan Centaur design, and an incident in March on a test article for the rocket’s Centaur stage ended up in a fireball that damaged the company’s test stand at NASA’s Marshall Space Flight Center in Huntsville, Alabama.

Despite Bruno saying damage was not that bad at the stand, he stated ULA won’t fly Certification-1 until an investigation into that incident is complete.

So ULA needs that in hand along with a successful test firing at Cape Canaveral before it will fly.