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Service bulletin SSM 51821

A bulletin is a manufacturer's communication to its dealers, as NHTSA indexes it. The summary is NHTSA's. The document is the manufacturer's: read it at NHTSA. A bulletin is not a recall, and listing a model year does not mean every vehicle of it is concerned.

The communication, as NHTSA indexes it (as of 2026-10-19)
TypeService bulletin
NHTSA id10240575
Published by NHTSA2023-08-10
Manufacturer's numberSSM 51821
Manufacturer's date2023-08-01
NHTSA componentPOWER TRAIN:TRANSFER CASE (4-WHEEL DRIVE); SUSPENSION:REAR:AXLE:NON-POWERED AXLE ASSEMBLY; POWER TRAIN

NHTSA's summary

Some 2020-2021 Explorer/Aviator/Police Interceptor Utility vehicles equipped with AWD may exhibit a binding or shudder sensation while turning and/or while accelerating from low speeds. The concern may also be described as a grinding and/or chatter noise. If the Workshop Manual (WSM) leads to replacement of the rear axle, inspect the differential for damage and differential fluid for metallic chunks in the fluid. Note that some metallic flake in the fluid and/or discoloration is normal and the rear axle should not be replaced for it. If no damage is found to the rear differential, it is not recommended to replace the rear axle assembly. Instead, refer to the Symptom Charts in WSM, Section 307-07A for further diagnosis of the transfer case.

The document

The document is the manufacturer's. This site does not hold it. NHTSA files a document at one of these two addresses; we have not asked NHTSA which.

Components

Suspension, Transmission and driveline, Transfer case

Vehicles listed
FORD EXPLORER2020, 2021
FORD EXPLORER POLICE INTERCEPT2020, 2021 (the catalog's model; the bulletin names a series or trim of it)
LINCOLN AVIATOR2020, 2021

Listing a model year does not mean every vehicle of it is concerned: the document says which.