Diono does not sell this seat on Amazon. See its own site for photos.

Diono Monterey 5iST FixSafe High-Back Booster

A high-back booster seat with one mode: high-back booster 40 to 120 lb, 38 to 63 in.

At a glance

Fits children from 40 to 120 lb
Up to 63 in tall
High-back booster only
8-year usable life

Not sold on Amazon

Diono lists this seat on its US site. Buy from Diono or an authorized retailer, and confirm the exact model number before you buy.

Diono product page Manual

Specifications checked against Diono’s US site and manual on September 17, 2026. Not hands-on tested. How I research

Who is the Monterey 5iST FixSafe High-Back Booster for?

The Diono Monterey 5iST FixSafe is a folding high-back booster for children 40 to 120 lb and 38 to 63 inches tall, whose ears sit below the top of the raised head support or vehicle headrest. It needs a lap and shoulder belt, attaches with rigid lower anchor connectors, has no backless mode, and expires 8 years after purchase.

Reasons to consider it

Rigid ISOFIX connectors whose indicator turns from red to green when they click onto the lower anchors
A fold lever plus carry straps that store in a compartment on the back of the seat
12 head support positions for setting the red shoulder belt guide at least 1 cm above the shoulders
A 120 lb and 63 inch upper limit with an 8 year life

Things to weigh up

High-back only; Diono says not to use it backless
Diono lists the seat at 18.7 lb and 17 in wide
Not certified for aircraft, because plane seats have no shoulder belt
Cannot be used with lap-only, motorized or door-mounted vehicle belts

My take on the Monterey 5iST FixSafe High-Back Booster

The Monterey 5iST suits a family that wants a high-back booster that folds and goes on a parent's back with built-in carry straps. The trade-off is that it never becomes a backless booster, and Diono lists it at 18.7 lb. Against the Monterey 6XT and 6DXT SafePlus, it uses rigid ISOFIX connectors with a red-to-green indicator instead of adjustable LATCH straps, and unlike the 6DXT it comes with no side impact pod.

A research summary of Diono’s published specifications and manual, read October 7, 2026. Not a hands-on test or a crash-safety rating.

Who the Monterey 5iST FixSafe High-Back Booster fits

This seat covers one stage. Diono gives it a 8-year life from the date of manufacture, so check the date label before using a hand-me-down.

High-back booster40 to 120 lb · 38 to 63 in: The seat’s own back and belt guide route the vehicle belt across the shoulder, and the sides give the child something to lean on when they sleep.

A school-age child with a backpack beside an empty high-back booster

What are the Monterey 5iST FixSafe High-Back Booster weight, height and age limits?

Diono Monterey 5iST FixSafe High-Back Booster limits by mode
Mode	Child weight	Child height	Minimum age	Conditions
High-back booster	40 to 120 lb	38 to 63 in	Not published	—

A child must meet every limit for the mode they use. The labels on your seat and its manual are the final word.

Monterey 5iST FixSafe High-Back Booster specifications

Seat weight: Not published
Width: Not published
Usable life: 8 years
Aircraft use: Not certified for aircraft

An empty car seat installed on a back seat, seen through an open car door

Will it fit your car?

Diono does not publish a width for this seat, so measure your back seat and compare it with the seat in a store before buying.

Check your vehicle manual for lower anchor positions and any head-restraint rules.
Rear-facing seats need front-seat room: try the install behind the driver before you commit.
Not certified for aircraft use: plan on gate-checking it or using a different seat to fly.

What to know before you live with it

Installing with ISOFIX or the belt alone

Diono recommends the ISOFIX connectors whenever the vehicle has lower anchors. You press the release button on the front to extend them, line them up with the anchors and push until each indicator changes from red to green. Then you push the seat back until it touches the vehicle seat. A gap is acceptable as long as the head support touches the vehicle seat back or headrest. If the connectors interfere with the belt, skip them, but the seat must then stay belted in even when empty. In a center position it must not rock from side to side.

Source: Diono Monterey 5 (5iST FixSafe) manual (PDF)

Routing the belt and checking the buckle

The child sits upright and all the way back. You squeeze the lever on the back to move the head support until the red shoulder belt guide is at least 1 cm above the shoulders, then feed the shoulder belt behind the metal clip in that guide. The lap belt rests in both lower belt guides, flat and untwisted, and the shoulder belt must not touch the neck. The manual also asks you to check the buckle: it should sit outside the lower belt guide. If it is pulled across the guide into the seat, choose another seating position.

Source: Diono Monterey 5 (5iST FixSafe) manual (PDF)

Folding, carrying and the cup holder

The seat folds with the seat fold lever, and the manual says the carry straps may only be used once it is folded. You attach the strap hooks to the top and bottom loops on the back, move the slider to adjust the fit and wear the seat on your back. The manual warns against using the straps if you have back, shoulder or neck problems. When not in use, the straps store in a compartment on the back of the seat. The cup holder pushes down onto a connector on the seat bottom and pulls up to remove. Do not use it for hot liquids or step on it.

Source: Diono Monterey 5 (5iST FixSafe) manual (PDF)

Taking the cover off and washing it

The headrest cover pulls out of the channels along the outside of the headrest. The main cover pulls out of the channels along the side walls, and the seat bottom cover comes off by undoing the hook and loop and elastic loops underneath. The care table says to machine wash the seat cover in cold water on a gentle cycle and dry it flat in the shade, with no bleach, ironing or dry cleaning. The shell and plastic parts are spot cleaned with warm water and mild detergent. To refit, tuck every plastic tab back into its channel before use.

Source: Diono Monterey 5 (5iST FixSafe) manual (PDF)

Is the Monterey 5iST FixSafe High-Back Booster recalled?

No campaign in the current NHTSA child-restraint data names this model family, and I have not run a separate review for it. Check your seat’s model number and date of manufacture before use.

NHTSA recall search · Recent car seat recalls

Monterey 5iST FixSafe High-Back Booster: common questions

What are the Monterey 5iST FixSafe weight and height limits?

The manual allows children who weigh 40 to 120 lb and are 38 to 63 inches tall. Their ears must also be below the top of the fully raised booster head support or vehicle headrest.

Diono Monterey 5 (5iST FixSafe) manual (PDF) (opens in a new tab)

Can the Monterey 5iST be used as a backless booster?

No. Diono says the 5iST does not have a detachable back and should not be used backless. It lists the Monterey 4DXT, Monterey 2XT, Cambria 2 and Cambria 2XT as its high-back boosters with a detachable back.

Diono support: Monterey 5iST FixSafe (opens in a new tab)

How wide and heavy is the Monterey 5iST FixSafe?

Diono lists it at 17 in wide, 15 in deep and 24.8 in tall, with a seat weight of 18.7 lb. It says its boosters fit 2-across in most cars and that each vehicle is different, so check the vehicle owner's manual before trying more.

Diono support: Monterey 5iST FixSafe (opens in a new tab)

Can I take the Monterey 5iST on a plane?

No. The manual says it is not certified for use in aircraft, because aircraft seats do not have the shoulder belt this booster needs.

Diono Monterey 5 (5iST FixSafe) manual (PDF) (opens in a new tab)

When does the Monterey 5iST expire?

The manual says not to use it once it is over 8 years from the date of purchase, or if any part is damaged, missing or the straps are cut or frayed. In the U.S., it should also be replaced after a moderate or severe crash.

Diono Monterey 5 (5iST FixSafe) manual (PDF) (opens in a new tab)

Answers refer to the model and manual version on this page. Check the instructions supplied with your seat.

Before you buy

Weigh and measure your child. The limits above are the manufacturer’s, and a child has to meet all of them for the mode they will use.
Confirm the exact version. Colors, retail exclusives and model numbers change limits more often than you would expect.
Read the manual before you buy, not after. It is linked above and holds the fit rules, the install methods and the expiry.
Check recalls by model number and date of manufacture, and register the seat with Diono so you hear about the next one.

Sources

Other Diono booster seats

Model	Top child weight	Seat weight	Width	Usable life
Monterey 5iST FixSafe High-Back Booster (this seat)	120 lb	—	—	8 years
Connect3 R 2-in-1 High-Back Booster	120 lb	—	—	8 years
Connect3 R Backless Booster	120 lb	—	—	8 years
Connect3 RXT 2-in-1 High-Back Booster	120 lb	—	—	8 years
Connect3 RXT Backless Booster	120 lb	—	—	8 years
Cambria 2 2-in-1 High-Back Booster	120 lb	—	—	8 years
Cambria 2XT 2-in-1 High-Back Booster	120 lb	—	—	8 years
Everett NXT High-Back Booster	120 lb	—	—	8 years
Monterey 2XT 2-in-1 High-Back Booster	120 lb	—	—	8 years
Monterey 4DXT 2-in-1 High-Back Booster	120 lb	—	—	8 years
Monterey 6XT SafePlus High-Back Booster	120 lb	—	—	8 years
Monterey 6DXT SafePlus High-Back Booster	120 lb	—	—	8 years
Solana Backless Booster	120 lb	—	—	8 years
Solana 2 Backless Booster	120 lb	—	—	8 years