Fix ERROR_DS_SUB_CLS_TEST_FAIL 0X000020C7 in AD

Cause #1: Schema conflict between domain controllers

This is the most common cause. You'll see ERROR_DS_SUB_CLS_TEST_FAIL (0X000020C7) when one domain controller has a newer schema version than another, and replication tries to apply an object that references a class or attribute the receiver doesn't recognize yet.

This happens most often after you've extended the schema — maybe you installed Exchange, a third-party app, or ran a schema update manually. One DC got the update, the other didn't, and now replication chokes.

How to confirm this is the problem

Open an elevated command prompt. Type this:

repadmin /showrepl

Look for the source DC that's failing. You should see the 0X000020C7 error next to a specific directory partition. Then run:

repadmin /showobjmeta <source-DC> <DN-of-failing-object>

If you see an attribute with schemaIDGUID that's missing on the receiving DC, that's your culprit.

Fix it: force schema sync

1. On the receiving DC (the one getting the error), open an elevated command prompt.
2. Run repadmin /syncall /AdeP. This forces replication of all partitions including schema. Wait for it to complete — it can take a few minutes.
3. Check the event log for Event ID 1925 or Event ID 1988 that confirm the schema update applied.
4. After that, run repadmin /replicate <target-DC> <source-DC> DC=domain,DC=com to manually retry the failed replication.
5. Verify with repadmin /showrepl — the error should be gone.

What if that doesn't work? Sometimes the schema conflict is deeper. You might need to demote the receiving DC and promote it again. That's drastic but reliable. I've had to do it twice in 10 years. Don't jump to it unless you've exhausted everything else.

Cause #2: Corrupt or orphaned object class in Active Directory

Less common but nastier. This happens when an object's objectClass attribute references a class that doesn't exist in the schema, or the class hierarchy got broken. You'll see this error on a specific object, not across the board.

Trigger scenario: You deleted a schema class or attribute but left objects that still reference it. Or a third-party tool messed up the schema.

How to find the orphaned object

Run this command on a healthy DC (preferably the schema master):

repadmin /showrepl * /csv > C:\repl.csv

Open the CSV, look for the object with the error. Note its distinguished name (DN). Then run:

dsquery * <DN> -attr objectClass

You'll see the class list. Compare it to the schema. If one class doesn't exist in CN=Schema,CN=Configuration,DC=domain,DC=com, that's the problem.

Fix it: repair or remove the broken object

1. Open ADSI Edit (from Tools in Server Manager). Connect to the Schema naming context.
2. Navigate to CN=Schema,CN=Configuration,DC=domain,DC=com. Find the missing class — if it's truly gone, you'll need to recreate it using the same schemaIDGUID. Export the class from a backup DC if you have one.
3. If the object itself is corrupt and can't be fixed, delete it. Use dsrm <DN> with care. Make sure you have authoritative restore ready.
4. After cleanup, run repadmin /syncall /AdeP again.

Warning: Don't delete objects willy-nilly. If it's a user or group, you'll lose access. Only delete if you're certain it's orphaned and unrecoverable.

Cause #3: Time skew between domain controllers

Yes, this old chestnut. If the source DC's clock is more than 5 minutes off from the receiving DC, the sub-class test can fail because timestamps on schema objects don't line up. This is rare with modern NTP setups but still shows up in labs or misconfigured environments.

Check time sync

Run this on both DCs:

w32tm /query /status

Compare the Last Successful Sync Time and Source. If one DC points to an external source and the other to the domain hierarchy, that's your mismatch.

Fix it: align time

1. On the DC with the wrong time, run w32tm /config /manualpeerlist:<correct-NTP-server> /syncfromflags:manual /reliable:yes /update. Use the PDC emulator as the source.
2. Then net stop w32time && net start w32time followed by w32tm /resync.
3. Verify with w32tm /query /status. The offset should be under a second.
4. Now retry replication.

I've seen time skew cause weirder errors than 0X000020C7. Always rule it out first if you're in a hurry.

Quick-reference summary