Updated GM Bulletin # 25-NA-185 : Engine Coolant Control Valve Failures on 3.0L Duramax (Dec 4, 2025)

Greg Nelson
2 days ago
4 min read

Plus: Understanding the LM2 & LZ0 Special Coverages (N252508341 & N252508340)

GM has released a revised version of # 25-NA-185, consolidating previous coolant control valve guidance into a single bulletin and adding mandatory ECM-calibration steps. At the same time, GM has active Special Coverage programs for specific LM2 and LZ0 models, which function as extended warranties for the coolant control valve under defined conditions.

This article explains all three programs—what changed, who is covered, and what owners should expect when diagnosing and repairing coolant control valve failures.

Overview of Updated Bulletin # 25-NA-185

Release: Revised December 4, 2025

Applies to: 2021–2026 3.0L Duramax (LZ0 and LM2) across Chevrolet and GMC 1500 trucks and 2025 full-size SUVs.

What the Bulletin Addresses

A vehicle may experience a failed engine coolant control valve (CCV) resulting in improper coolant routing, long warm-up times, reduced cabin heat, or related DTCs. GM instructs technicians to:

Replace the coolant control valve
Verify and install the latest ECM calibration

This calibration requirement used to be in a separate bulletin—GM has now merged procedures so the mechanical and software updates are included together.

What Changed in the December 2025 Revision

This update is effectively a consolidation of prior guidance. Key additions include:

LM2 models added
Expanded year coverage through 2026 in certain SUV applications
ECM calibration verification required for every repair
Updated warranty labor times and operations

The bulletin does not list a single root cause. GM states the failure “may be due to multiple conditions.” Field evidence shows internal breakage, sticking, or electrical motor faults inside the valve assembly.

Warranty Coverage Under # 25-NA-185

This bulletin is not a Special Coverage. It applies only when the vehicle is within the Powertrain Warranty period. Labor operations differ by model year and truck vs SUV, ranging from 2.8 to 3.8 hours for CCV replacement plus calibration verification.

ECM reprogramming uses published labor time when required.

How # 25-NA-185 Relates to the Special Coverages

GM currently has two active Special Coverage programs that also address coolant control valve failures:

N252508341 – LM2 engines (2021–2023 SUVs + 2021–2022 trucks)
N252508340 – LZ0 engines (2023 trucks only)

These Special Coverages act like extended warranties on the valve for 15 years or 150,000 miles, but apply only when diagnostics show the valve is the cause of a specific failure.

This means:

Some owners will be covered under #25-NA-185 (standard powertrain warranty).
Others may be covered under one of the Special Coverages, depending on VIN.
Many owners of out-of-warranty vehicles may still qualify under Special Coverage even if powertrain warranty has expired.

VIN checks are mandatory. Only GM’s IVH → Applicable Warranties will confirm whether a specific vehicle is listed.

Special Coverage N252508341 (LM2 3.0L Duramax)

Release: September 2025

Applies to LM2-equipped:

Chevrolet Silverado 1500 (2021–2022)
Chevrolet Suburban (2021–2023)
Chevrolet Tahoe (2021–2023)
GMC Sierra 1500 (2021–2022)
GMC Yukon (2021–2023)
Cadillac Escalade (2021–2023)

What the Special Coverage Does

If diagnostics confirm the coolant control valve failed an onboard diagnostic test, GM replaces the valve at no charge within:

15 years or 150,000 miles(from original in-service date, whichever comes first).

Symptoms GM Calls Out

MIL/Check Engine Light
Cooling fans staying on after key-off
Diagnostic trouble codes relating to coolant flow(GM does not list specific DTCs; P1098 and P26BB are common in field reports.)

Dealer Procedure

Perform SI diagnostics
If diagnostics verify the valve is the root cause, replace:
- Engine Coolant Flow Control Valve (P/N 40009644)
- Oil Level Indicator Seal (P/N 12670252)
- Refill with correct coolant (12346290 US / 10953464 CA)

Parts are not to be stocked; they are ordered per vehicle.

Reimbursement

Owners who paid for a related repair before September 16, 2025 may request reimbursement. Documentation is required.

Special Coverage N252508340 (LZ0 – 2023 Only)

Release: September 2025

Applies to:

2023 Chevrolet Silverado 1500 (LZ0)
2023 GMC Sierra 1500 (LZ0)

This Special Coverage is limited to one model year and does not list 2024–2025 LZ0 trucks.

Coverage Window

Same as LM2: 15 years or 150,000 miles.

Symptoms

MIL/Check Engine Light
DTCs related to coolant flow (commonly P1098)
Cooling fans running after shutdown
Possible coolant leak at the valve

Dealer Procedure

If diagnostics confirm the valve:

Replace Engine Coolant Flow Control Valve (P/N 40009452)
Replace Oil Level Indicator Seal (P/N 12670252)
Refill coolant (12346290 US / 10953464 CA)

Parts are Customer Special Order (CSO). Supply is limited.

Reimbursement

Repairs performed before September 2025 may be eligible for reimbursement. GM sets a deadline of October 31, 2026 unless state law allows more time.

How to Determine Your Coverage

Because GM’s bulletins depend entirely on VIN eligibility, owners should request a dealer check:

IVH → Applicable Warranties

Possible outcomes:

Your vehicle is inside Powertrain Warranty → #25-NA-185 may apply
Your vehicle is outside Powertrain Warranty but listed under a Special Coverage → N252508340 or N252508341 may cover the repair at no cost
Your vehicle is not listed → repair follows standard warranty or customer-pay rules

Many LM2 and LZ0 owners will be pleasantly surprised to learn they have 15-year/150k-mile protection.

What I Would Do (Owner Checklist)

Scan for codes if you see a Check Engine Light or your cooling fans run after key-off.
Request an IVH check for both Special Coverages and bulletin applicability.
Let the dealer diagnose it. GM covers the repair only if diagnostics confirm the valve.
Expect a parts order. Supply is controlled and not stocked in volume.
If you already paid for this repair, begin reimbursement with GM using your repair order and proof of payment.

Why GM Has Multiple CCV Documents

GM is now juggling:

A standard service bulletin for 2021–2026 models (#25-NA-185), requiring replacement + ECM update
LM2 Special Coverage (N252508341)
LZ0 Special Coverage (N252508340)

This creates overlap, but the purpose is clear:

#25-NA-185 standardizes the repair and ensures vehicles get the updated software
N252508341 and N252508340 extend coverage for older or specific models with higher failure reports
All three programs help create a consistent diagnostic and repair path across LM2 and LZ0 trucks and SUVs

If additional part supersessions or calibration updates emerge, this article will be updated accordingly.

GM Bulletin # 25-NA-185 – Engine Coolant Control Valve Failure (Dec 4, 2025 revision)
GM Special Coverage Bulletin N252508341 – LM2 Coolant Control Valve Replacement (September 2025)
GM Special Coverage Bulletin N252508340 – LZ0 Coolant Control Valve Replacement (September 2025)