A320 pilots reported the autopilot failed to capture the RNAV inbound course and contributed to the unstable approach.
Synopsis
A320 pilots reported the autopilot failed to capture the RNAV inbound course and contributed to the unstable approach.
Narrative
On arrival to ZZZ being vectored for the RNAV (GPS) Y XXR everything was progressing well. When given a decent the PF said Open Descent (OP DES) wouldn't engage and was using V/S. In retrospect this was the first distraction. When cleared for the approach; the approach was armed and FINAL APP Flight Mode Annunciator (FMA) was cross checked. The aircraft flew through the final approach course which was the second distraction. The PF disconnected the autopilot simultaneous to ZZZ giving us a heading to reintercept. The PF over banked and our altitude began to increase. PM called 'Check Path' and path stabilized. ZZZ canceled our approach clearance; told us to level at 4;000 ft and gave us a vector for another approach. Altitude was decreasing to 3;800 which was called out by ATC as correction was being applied. Airspeed was slowing as Auto Thrust was previously disconnected with Autopilot by PF. PIC assumed PF duties. Aircraft was stabilized. We were given a decent and heading. PIC was unable to duplicate problem engaging OP DES. Once stabilized and FINAL APP was engaged and verified to be working correctly; PIC resumed PM duties followed by a normal landing.
Second reporter narrative
During final short vector to intercept final approach course the auto pilot did not capture and proceeded inbound or descend . However; we did have final app on the Primary Flight Display (PFD) Pilot intervened to correct and the Approach Controller directed a cancel of approach clearance And told to maintain an altitude of 4000 feet. I checked my altitude at 3800 and corrected. Returned to the same approach and completed. Safe landing.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.