I think that the cause of its malfunction lies in this.
I suspect that the machine's malfunction is due to ○○.
ご質問ありがとうございます。
① "I think that the cause of its malfunction lies in this."=「不具合はこれが原因だと思います。」
② "I suspect that the machine's malfunction is due to ○○."=「機械の不具合は○○が原因だと思っています。」
追加:"I think I finally figured out what the problem is."=「やっと問題が何か分かったと思います。」
ご参考までに。