Parts Inventory Ledger doesn't match parts stock valuation?

Usual Causes:

1. Postings made to the Parts Inventory Ledger unrelated to parts orders/invoices. Usually where creditors invoices, or journals get mistakenly posted to this account it will cause the two accounts to deviate from each other. 

Easiest way to check this is to do a ledger enquiry on the Parts Inventory account and then set the Transaction type filter to show "Journal" and then run it again with the filter set to "Creditors Invoice", this should then show only those types of transactions that have occurred.

2. Parts stock going into negative can cause the Inventory value to not match the parts stock valuation report. When parts inventory goes into negative it will decrease the ledger value by the current average value but when an order is made for the part the ledger value will adjust up by the value of the order, which can cause a difference in the values if the order value and the previous average value are different. The further into negative the part goes, or the greater the difference in value between the average and the order the larger the difference can become.

Adjusting them to match:

If you're satisfied that the parts stock valuation report is accurate and want the ledger to match then you can do a journal Debiting/Crediting the Parts Inventory Account(Asset) on one side and then usually the other side will be Crediting/Debiting the Parts Inventory Adjustment Account(Cost of Sales), this will adjust the Inventory Up/Down by that amount and put the amount to your Cost of Sales.