I started the lockloss investigation on 2025/03/05 8:58 UTC ~ 20:45 UTC. Previous lockloss investigation was posted in klog32803.
During this period, there were 18 lockloss. The plots are summarized in wiki.
 

Saturation of OMC PD

Just before the lockloss, we observed the saturation of both OMC PD. In the previous investigation, we saw similar saturation with reducing the AS 17 (which indicates the reducing of the optical gain). But, this time, we did't see such reduction. The AS17 signal looks healthy.
Although the OMC PD signal was oscillated with 20 Hz in most lockloss, the amplitude was not increased. As I heard from Ushiba-san, this frequency came from the suspension and is not related to the lockloss. Figure 1 and 2 show the example.

For futher investigation, I checked channels related to OMC LSC control (Figure 6). One finding is that the typical value of K1:OMC-TRANS_DC_DIFF_OUTPUT is -0.45, which is different from the previous typical value. For example, the value of 12/17 is -0.26.
After reparing the OMC PD B, we did't perform the dark noise subtraction. We need to do it in future. But, as chated with Ushiba-san, this will be not the cause of the lockloss.

Although I checked the channels to monitor the mirror motion, I didn't find the strange motion. I plan to compare the spectrum of OMC PD B is same as previous.

Transient change of ISS feedback

Probably this is not the cause of the lockloss. But, I found the sudden drop of ISS feedback signal (K1:PSL-ISS_FIRST_SERVO_AOM_MON_OUT_DQ) and K1:LAS-POW_PMC_OUT_DQ, even though the error signal of ISS was not changed.
Figure 3 and 4 show the examples.
 

1Hz seismic motion made the IMC lockloss

One lockloss was caused by the 1 Hz seismic motion. That makes the 4 Hz oscillation of the feedback signal of IMC length and IMC lockloss, as shown in Figure 5.