TBS_E_SERVICE_START_PENDING (0X8028400B) Fix Guide

What's Happening Here

You're seeing error 0X8028400B (TBS_E_SERVICE_START_PENDING) because the Trusted Platform Module (TPM) Base Services service can't finish starting up. The TBS service acts as a middleman between Windows and your TPM chip. When it gets stuck in "start pending," it means Windows thinks the service should be running, but it's hung while trying to initialize. This usually happens right after a BIOS update, a TPM firmware update, or a Windows feature update that messed with how the TPM is configured.

I've seen this most often on Dell and Lenovo business machines with TPM 2.0, but it can hit any PC. The service will sit there for 30 seconds to 2 minutes, then time out with this error. Don't waste time restarting the service — that won't fix the underlying problem.

Cause #1: Corrupted TPM Driver in Device Manager

This is the real fix nine times out of ten. Windows Update sometimes pushes a generic TPM driver that doesn't play nice with your specific TPM chip. You need to force Windows to re-detect and install the correct driver.

Step-by-step Fix

1. Press Win + X and select Device Manager. You'll see a list of all your hardware.
2. Look for Security devices and expand that section. You should see something like "Trusted Platform Module 2.0" listed there.
3. Right-click that TPM entry and choose Uninstall device. A warning dialog pops up — check the box that says Delete the driver software for this device. Then click Uninstall.
4. Wait for the uninstall to finish. Your screen might flicker or go dark for a second — that's normal.
5. Restart your computer. Don't skip this — a restart forces Windows to pick up the correct driver during boot.
6. After the restart, open Device Manager again. The TPM entry should be back under Security devices. If it isn't, go to the Action menu and click Scan for hardware changes. It'll reappear.
7. Now test: open an admin command prompt (Win + X, then Command Prompt (Admin) or Terminal (Admin)). Type net start tbs and press Enter.

After doing this, you should see "The TBS service is starting..." followed by "The TBS service was started successfully." No more error.

Cause #2: TPM is Disabled or Configured Wrong in BIOS

The second most common reason — someone (or a BIOS update) turned off the TPM in the firmware settings. Or it's set to "TPM 1.2" when Windows 11 wants 2.0. The TBS service can't start if the hardware it talks to isn't ready.

How to Check and Fix BIOS Settings

1. Restart your PC. As it boots, press the key to enter BIOS/UEFI setup. That's usually F2 on Dell or Lenovo, Del on most desktops, F10 on HP, or Esc on some Asus boards. Watch the screen for the prompt.
2. Inside BIOS, look for a section called Security or Trusted Computing. It might also be under Advanced on some motherboards.
3. Find the setting for TPM State or TPM Device. It should be Enabled. Some BIOS versions call it "TPM Availability" — set that to Available or Enabled.
4. Check the TPM Firmware Version setting if it's there. Windows 11 needs TPM 2.0. If it's set to 1.2, change it to 2.0.
5. Save and exit (usually F10). Let the machine boot fully into Windows.
6. Once back in Windows, open Device Manager and verify the TPM is listed under Security devices. Then test with net start tbs in an admin command prompt.

If you still get the error, go back into BIOS and look for a setting called Clear TPM or Reset TPM to Factory Defaults. Use it — this wipes the TPM's stored keys (safe for most users, but backup any BitLocker recovery keys first). Then save, exit, and test again.

Cause #3: Corrupted TBS Service Registry Entry

Less common, but when the registry key for the TBS service gets scrambled — usually after a bad uninstall of security software — it can't load the right DLL. The error code 0X8028400B will stick around even after fixing the driver and BIOS.

How to Restore the Registry

1. Open the Registry Editor: press Win + R, type regedit, hit Enter. Click Yes when UAC asks.
2. Navigate to this exact path: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\TBS
3. In the right pane, look for a value called ImagePath. Double-click it and make sure its data is %SystemRoot%\System32\tbs.dll. If it's missing or points to something else (like a leftover antivirus DLL), change it to that exact string.
4. Also check the Start value. It should be 3 (manual start). If it's 4 (disabled) or 2 (automatic), double-click it and set it to 3.
5. Close Registry Editor and restart your PC.
6. After the restart, open an admin command prompt and run net start tbs again.

If the service starts now, you're good. If not, there's one more thing to try: run sfc /scannow in an admin command prompt to check for system file corruption that might have damaged the TBS files.

Quick Table Summary

Start with the driver fix — that's the one that works for most people. If that doesn't do it, move to BIOS. The registry fix is last resort. You'll have this error licked before you know it.