Understanding the MMT 399 Error Code

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FAQs

What is the MMT 399 error code?

The MMT 399 error code indicates an issue with the transmission control module (TCM) in vehicles equipped with an automatic transmission system. It typically points to a faulty TCM that needs to be replaced.

What causes the MMT 399 error?

Common causes include faulty wiring, bad circuit board/connections, software glitches, or a failing TCM unit. Certain events like water damage can also trigger this error code.

How can I diagnose the MMT 399 error?

You'll need to scan the TCM for any stored Diagnostic Trouble Codes (DTCs). Checking TCM connector pins for damage, testing wiring, and monitoring live transmission data can also help diagnose the root issue.

Is the vehicle safe to drive with MMT 399 error?

It is typically not recommended to drive with this error code active. It indicates possible transmission problems that can lead to unsafe conditions or further vehicle damage when driving.

How much does it cost to fix MMT 399 error code?

Cost varies depending on the exact issue, but typically ranges $1000 - $2000 including parts like a replacement TCM, programming, and labor for error diagnosis and repair work.