If the debt has been fully satisfied, the installment agreement is cancelled automatically.
Otherwise, as a condition of your installment agreement, any refund due to you in a future year will be applied against the amount that you still owe, and, you must continue making your installment agreement payments as scheduled and in full.
You may call the IRS at 1-800-829-1040 (very long wait time) and speak with the agent to find out how much you still owe; although they will also send you a notice with the details.
Please see more details in below links:
If the debt has been fully satisfied, the installment agreement is cancelled automatically.
Otherwise, as a condition of your installment agreement, any refund due to you in a future year will be applied against the amount that you still owe, and, you must continue making your installment agreement payments as scheduled and in full.
You may call the IRS at 1-800-829-1040 (very long wait time) and speak with the agent to find out how much you still owe; although they will also send you a notice with the details.
Please see more details in below links: