General Motors has updated Service Bulletin #25-NA-185 to include 2027 model-year vehicles equipped with the 3.0L Duramax diesel. The latest revision continues to address coolant control valve failures, while the history of replacement part numbers and ECM programming updates shows how GM's repair approach has changed over the years.

On October 26, 2026, General Motors released Version 4 of Service Bulletin #25-NA-185, Engine Coolant Control Valve Issues. The update adds the 2027 model year to the existing bulletin, which was originally released in July 2025.

GM routinely updates service bulletins as new model years are introduced, so the addition of 2027 vehicles doesn't necessarily indicate that failures have already occurred on the newest trucks. However, the coolant control valve has gone through several part-number changes since the 3.0L Duramax was introduced, and the latest LZ0 replacement valve requires updated ECM programming.

Vehicles Covered Under Bulletin # 25-NA-185

The bulletin applies to vehicles equipped with the LM2 or LZ0 3.0L Duramax diesel. GM also identifies both the Limited/LTD and New configurations of the 2022 Silverado and Sierra.

Although the LM2 was introduced in the 2020 Silverado and Sierra, the current version of this particular bulletin begins with the 2021 model year.

LM2 Coolant Control Valve Part-Number History

The original LM2 Duramax uses a different coolant control valve assembly than the later LZ0. GM parts catalogs identify several previous part numbers that have been superseded by the current replacement, 40009644.

These numbers are listed within the same supersession family, although the catalog information does not establish the exact order in which each revision was introduced.

A new part number doesn't always mean the component was redesigned. GM may supersede a part because of a supplier change, manufacturing update, or other parts-related change.

LZ0 Coolant Control Valve Part-Number History

The LZ0 was introduced in the Silverado and Sierra for the 2023 model year, followed by the full-size SUVs for 2025. Its coolant control valve uses a separate part-number family.

The current replacement, 40009452, is a second-generation variation of the LZ0 coolant control valve. Installation requires the appropriate updated ECM calibration.

The valve replacement procedure therefore includes checking the ECM software version and updating it when necessary. GM specifically includes that instruction in the latest revision of bulletin # 25-NA-185.

GM Has Also Updated ECM Programming

GM previously released ECM update 661.30, associated with field action N25-251385, for certain 2023 Silverado and Sierra trucks. The update included diagnostic changes intended to help extend coolant valve life.

The ECM controls coolant valve operation and monitors its performance. Changes to the calibration can affect how the system operates and responds to faults, although the available update description does not explain the specific changes made to improve valve longevity.

GM's current repair procedure calls for the replacement valve to be installed and the ECM calibration checked. If newer software is available, the ECM is to be reprogrammed.

LM2 and LZ0 Coolant Control Valve Failures

I've replaced coolant control valves on both the LM2 and LZ0 Duramax engines, and the failures I've encountered have been somewhat different.

On the LM2, most of the failures I've seen have involved the internal gears and motor mechanism. On the LZ0, I've found valves with broken internal components.

I've also noticed that coolant control valve failures tend to become more common during the winter months. Cold temperatures may contribute to the failures, but I haven't established a specific cause for that seasonal pattern.

GM's bulletin doesn't identify a single failure mechanism, stating only that multiple conditions may be responsible.

So far, the failed valves I've encountered have all been original factory-installed components. I haven't seen a replacement valve come back with the same failure, although that doesn't necessarily mean the newer parts have eliminated the problem.

GM's Current Repair Procedure

Bulletin #25-NA-185 instructs technicians to replace the failed coolant control valve and verify that the ECM has the latest available calibration.

GM provides the following warranty labor times:

ECM reprogramming, when required, is listed under a separate labor operation using the published labor time.

GM has also issued special coverage programs for certain LM2 and LZ0 coolant control valve failures. These programs have their own eligibility requirements and should not be confused with the broader vehicle coverage listed in the technical service bulletin.

What About the 2027 LZ0?

The latest revision adds 2027 vehicles to the existing bulletin, and GM's parts catalog lists the current LZ0 replacement valve, 40009452, for at least some 2027 applications.

The 2027 LZ0 has received several engine updates, including changes to the pistons, piston rings, turbocharger operation, and emissions-related components. GM has not identified a new coolant control valve design in this bulletin.

The current replacement part listing doesn't necessarily establish which valve was installed during 2027 vehicle production. A factory-installed component may have a different production number than the service replacement listed in the parts catalog.

For now, the October 2026 bulletin continues the same repair instructions into the 2027 model year. Whether the latest valve and programming revisions have addressed the failures will become clearer as more vehicles accumulate mileage.

References: GM Service Bulletin # 25-NA-185, Version 4, October 26, 2026; GM Special Coverage N252508340 and N252508341; ECM Update 661.30 / N25-251385; GM replacement parts catalog.

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