Transmission drops a gear on the interstate, and the rear kicks like it hit black ice. That’s the nightmare behind GM’s latest recall. But some owners are stuck wondering, was it that defect, or just the return of the long-hated 8-speed shudder?
Two problems. One is a federal safety recall: NHTSA 24V-797 / GM N242454440, targeting late-model diesel trucks. The other’s Hydra-Matic hell, years of harsh shifts, converter slip, and fluid roulette. Both feel like transmission failure, but play out very differently once the dealer’s involved.
This guide draws the line. Which VINs are recalled, what GM’s fix does (and doesn’t) solve, and how owners are bridging the gaps with TSBs or DIY solutions. No guesswork, just a straight path from failure to fix.
1. The recall that actually matters: 24V‑797 (GM N242454440)
Who’s actually in the crosshairs
This recall targets 2020–2022 Silverado 1500, 2500, and 3500 diesels, plus diesel-powered GMC trucks and SUVs. Gas models are off the hook.
Roughly 461,839 vehicles are flagged in the U.S. GM issued a stop-sale order on October 24, 2024. Until the recall is fixed, dealers can’t sell or even demo these trucks. Owner letters started rolling out in December.
What’s breaking inside the transmission
The trouble starts with the control valve in the gearbox. As it wears, hydraulic pressure drops, and gear changes turn rough. Worst-case scenario? An 8th-to-lower downshift at speed that locks the rear wheels. That’s not just a bump, it’s a crash risk, and NHTSA calls it a clear safety defect.
What dealers actually do, software first
The fix starts with a software flash to the TCM. It watches valve performance and can catch wear nearly 10,000 miles before it turns dangerous.
If things go south, the software locks the truck into 5th gear, flips on the MIL, and shows “reduced propulsion” on the dash. That buys you time to get it repaired before the rear wheels seize. Whole visit takes about 45 minutes. No charge.
There’s also a special coverage program. If the valve problem shows up within 15 years or 150,000 miles, GM foots the bill for hardware repair. If you’ve already paid out of pocket, you may be eligible for reimbursement.
What it feels like behind the wheel
Hard downshift followed by the rear dragging? You’re probably in recall territory. After the update, if your truck locks into 5th and starts flashing warnings, that’s not a glitch. That’s the system telling you the hardware’s wearing out, and the repair window just opened.
What to do right now
• Check your VIN at NHTSA and call the dealer with N242454440. Get the TCM flash done first.
• Watch for the 5th-gear cap, warning lights, or reduced propulsion alert. That’s your green light for free repairs.
• If the issue feels more like a slow P-to-D shift or a vibration at cruise, that’s a different beast; we’ll break down the Hydra-Matic problems next.
2. The 8-speed Hydra‑Matic problems that won’t quit
What drivers are actually feeling
Owners call it “the shudder.” Feels like rumble strips at highway speed. Others report harsh gear changes, sluggish response leaving stoplights, or a sudden jolt like getting rear-ended.
Most of these complaints trace back to 2014–2019 Silverados, but some symptoms linger past warranty. No recall here, just years of drivability headaches.
Why the 8-speed behaves this way
The heart of the problem is the torque converter clutch. Heat, fluid breakdown, and aeration eat away at the lining. Once the fluid degrades, the converter can’t lock up cleanly. Instead of a smooth transition, the drivetrain chatters.
Critics argue GM used a fluid that couldn’t hold up under real-world stress. Over time, it breaks down and sets off the classic slip-grab “shudder” owners dread.
The legal storm building around it
Class actions like Shelton v. General Motors LLC claim GM knowingly sold trucks with weak 8L90 and 8L45 units. The accusation? GM pushed cheap fluid swaps instead of replacing bad parts.
Court documents highlight thousands of complaints, arguing these transmission quirks raise crash risk, even without a federal recall. Some judges are letting the cases proceed, forcing GM to defend its fix-first strategy.
Why it still matters even if you’re not recalled
No recall doesn’t mean no problem. And if you’re out of warranty, don’t expect a free fix. Dealers lean on TSBs and goodwill calls.
GM’s standard solution is a full fluid exchange with Mobil 1 LV ATF HP, but that only helps if you catch it early. Some owners say the vibration disappears, others report it comes back by the next oil change.
3. GM’s usual routine won’t cut it
TSBs aren’t recalls, and that changes everything
A TSB is a suggestion, not a demand. Dealers follow it to fix repeat complaints, but coverage depends on warranty status or goodwill. Recalls, by contrast, are federal and mandatory.
Most Silverado shift issues fall under TSBs, not recalls, so the fix depends on what you’re feeling and whether it matches a bulletin.
The blueprint for fixing the shudder
GM’s go-to fix for the rumble-strip shudder is TSB 18‑NA‑355. It calls for a full fluid exchange,20 quarts of Mobil 1 LV ATF HP, using the DT-45096 machine.
This isn’t a power flush. No fluid mixing. No additives. Technicians have to confirm the symptom first, then retest after the procedure to verify it worked.
Delay when shifting out of Park after a cold soak
When shifting from Park to Drive or Reverse takes longer than 3 seconds after an overnight sit between 10–24 hours, check TSB 20‑NA‑187.
It outlines how to time the delay, then runs a clutch re-learn procedure. If that fails, internal inspection is next. This bulletin separates normal lag from real faults.
Shake or hesitation during light throttle cruise
For steady-speed shake or chatter between 25 and 80 mph, TSB 19‑NA‑121 kicks in. It focuses on throttle calibration and drivability tuning.
Depending on the outcome, dealers may perform a software update or dig deeper. This one especially applies to 2019 Silverado 1500 models with the factory-updated fluid.
Don’t confuse “coverage” with “guaranteed”
A TSB may walk the service tech through a fix, but once the factory warranty ends, coverage is no longer automatic. Some advisors offer goodwill if your history backs it up.
But that’s separate from recall 24V‑797, which triggers a TCM update and optional valve coverage. Mix up the two, and you’ll spin your wheels.
Official repair paths that actually exist
| Type | ID | Affected scope | Condition owners feel | Factory action |
|---|---|---|---|---|
| Recall | 24V-797 / N242454440 | 2020–2022 Silverado diesels and related GMC, U.S. count about 462,000 | Harsh downshift, rare brief rear lock, safety risk | TCM software monitors valve wear, limits to 5th with MIL and reduced propulsion, special coverage for hardware if detected |
| TSB | 18-NA-355 | 8-speed Hydra-Matic applications, including Silverado, earlier model years | “Rumble-strip” shudder at cruise, harsh shifts | Exchange 20 quarts Mobil 1 LV ATF HP, verify with drive cycles, no additive mixing |
| TSB | 20-NA-187 | 8-speed apps | Delayed or harsh engagement P to D or R after soak | Time the delay, clutch learn, inspect internals if still slow |
| TSB | 19-NA-121 | 2018–2023 Silverado focus inside GM docs | Shake or shudder at 25–80 mph at steady speed | Calibration checks, software update, further diagnosis as needed |
What to say at the service desk
• Explain the problem like a driver. Mention speed, throttle, temperature, and how long it sat.
• Ask for the exact TSB. Say “18-NA-355” for shudder, “20-NA-187” for cold-start delay, or “19-NA-121” for steady-speed shake.
• If it’s a safety issue, say “I’m here for recall 24V‑797, code N242454440.” Get that campaign number printed on the RO.
4. The recall fix works, but don’t expect miracles
Why the software is still worth it
The TCM patch from 24V‑797 isn’t a basic reflash. It’s an early-warning system. It detects valve wear nearly 10,000 miles before the transmission becomes a safety hazard.
Instead of locking the rear wheels during a downshift, the system limits the truck to 5th gear, throws a MIL, and flashes “reduced propulsion.” That gives you time to get to the dealer before anything fails hard.
Where the recall ends
Here’s the bottom line: the software doesn’t fix the valve. The wear continues. Hydraulic pressure still drops. The metal part is still in there, waiting to cross the failure threshold. You won’t get a new valve unless the software catches enough wear to trip the limit.
That’s why GM paired this with special coverage, 15 years or 150,000 miles, for hardware replacement once the system flags it.
How it plays out on the road
Plenty of owners report their truck suddenly sticking in 5th gear with lights blazing across the dash. That’s not a bug. That’s the system doing its job, keeping you from a highway lock-up. But until the part is replaced, you’re dealing with capped performance and limited towing.
And keep this clear: the recall doesn’t touch 8-speed shudder, delayed engagement, or fluid-born drivability issues. That’s a different battle, and it lives in the TSB stack.
5. What you feel on the road, and what to do next
You don’t need a scan tool to know something’s off. The first sign comes through the wheel or the seat. Use this breakdown to zero in on the cause, and steer the fix the right way.
Read the symptom, follow the trail
| Road symptom you feel | Likely bucket | First move | Dealer code to cite | Expected outcome |
|---|---|---|---|---|
| Sudden harsh downshift at speed, brief rear drag | Recall 24V-797 | Run VIN, book the recall visit | N242454440 | TCM update installs, future risk downshift is blocked, special coverage if wear is detected later |
| Stutter like rumble strips at steady cruise, 25–80 mph | TCC shudder | Ask for verification drive, check TCC slip | TSB 18-NA-355 | Full exchange with Mobil 1 LV ATF HP, many improve, some recur and need deeper work |
| Hesitation or shake leaving a stop, light throttle | Calibration | Confirm latest TCM software | TSB 19-NA-121 | Shift logic updated, re-evaluate after drive cycle |
| 1–3 s delay going P to D or R after an overnight sit | Soak engagement | Time the delay, perform clutch learn | TSB 20-NA-187 | Learn procedure done, if still slow, inspect internals |
| Hot-day towing or mountain grades, shifts get sloppy | Heat management | Inspect for correct fluid, evaluate temps | None, service write-up | Add cooler where justified, confirm exchange history |
| Post-recall, truck stuck in 5th with MIL and message | Recall safeguard active | Schedule hardware repair under coverage | N242454440 on RO | Valve wear confirmed, hardware repaired at no charge under program limits |
Steps that actually get results
• Write it down: speed, throttle, road angle, ambient temp. Include tow weight if you were hauling.
• Run your VIN in two places: NHTSA for safety recalls, and GM Owner Center for special programs. Bring screenshots.
• Talk like a tech. Say, “I need recall 24V‑797, code N242454440.” Use bulletin codes for non-safety complaints.
• Don’t let them guess. Ask for the Mobil 1 LV ATF HP exchange, not a flush. Confirm 20 quarts, not 10.
• Retest on the same route. If it still stutters, bring notes and the RO number. Escalate from there.
6. Why owner-built fixes are taking over
Why forums don’t buy the factory line
GM claims its TSBs and fluid swaps fix most 8-speed complaints. But forums tell a different story. Many owners say the shudder fades, then creeps back.
Others report engagement delays that never fully vanish. The gap between the factory procedure and real-world results is what’s fueling the aftermarket scene.
Torque converter swaps that hold
Tired of repeat shudder, some drivers pull the trigger on a full torque converter replacement. Aftermarket units use stronger clutch linings that resist heat and fluid breakdown better than stock.
It’s not cheap; transmission-out labor adds up, but plenty swear it’s the only way to solve the root issue for good. Shops back it up with teardown pics of scorched factory parts under 60,000 miles.
Cooler installs to control fluid temps
Heat accelerates failure. In hot climates or under tow loads, 8-speed owners are adding auxiliary transmission coolers. Dropping fluid temps helps prevent aeration and slows clutch degradation.
This mod is already common in the 10-speed world, and 8-speed trucks are following suit, even without official backing from GM.
Additives: hype, hope, or hoax?
Products like Lubegard Platinum show up in a lot of DIY threads. Some owners say it smooths out lockup instantly. Others call it a temporary mask that delays the inevitable.
GM’s bulletins explicitly forbid any additives with Mobil 1 LV ATF HP, so this route could void goodwill coverage. Risky territory with mixed results.
Disabling AFM/DFM to smooth out shifts
Turning off cylinder deactivation is a popular workaround. Locking the engine into full V8 mode cuts down on load swings, which can clean up light-throttle shifts. But it sits in a legal gray area.
GM won’t honor warranty claims on trucks with the system disabled, and emissions rules make it a risky move.
Why these fixes even exist
Factory solutions don’t always restore trust. That’s why parts stores keep converters, coolers, and disablers in stock for trucks that shouldn’t need them. The demand says it all; owners aren’t convinced a TSB alone will make things right. When confidence breaks down, the wallet takes over.
7. Owner Action Plan That Actually Gets Results
No guesswork. Just a straight path from symptom to fix.
Run your VIN the right way
Start with NHTSA’s Recall Lookup, which shows open federal recalls tied to your exact VIN. Only safety-related campaigns are included.
Then go to the GM Owner Center, where you’ll find additional special coverages GM may offer outside of official recalls. Be sure to take screenshots; these can matter at the dealership counter.
Book the recall visit with precision
When calling the dealer, say: “I need recall 24V 797, GM code N242454440, TCM update.” Confirm that they list this as a safety campaign, and request that the RO (repair order) prints the campaign number on your pickup paperwork.
Understand what the update really does
The TCM flash allows the truck to detect valve wear up to 10,000 miles before it becomes dangerous. If that wear is detected, the vehicle limits itself to 5th gear, throws a MIL (Malfunction Indicator Lamp), and displays “reduced propulsion”, a safeguard to prevent a locked rear axle at highway speeds.
What to do if your truck locks into 5th
If your truck locks into 5th, don’t continue driving long-distance. Book another dealer visit and cite N242454440 again. Ask about the special coverage; hardware repair is covered free of charge if the software detects the fault within 15 years or 150,000 miles. Bring your first repair order so they can link your records.
No recall, but shudder or delay?
For cruise-speed vibration, request TSB 18-NA-355 and confirm the shop performs a controlled exchange using 20 quarts of Mobil 1 LV ATF HP and the DT-45096 tool. No flushes, no additives.
If you’re seeing cold-start delays when shifting from Park, refer to TSB 20-NA-187, which outlines timing specs, clutch learn values, and the procedure if the learn fails. For throttle hesitation or driveline shake, ask about TSB 19-NA-121, which addresses calibration and drivability updates.
Get paperwork that actually protects you
Your repair orders should show the exact bulletin or recall number; don’t accept vague notes like “TCM update.” If transmission fluid was changed, make sure the RO states: “Mobil 1 LV ATF HP – 20 quarts.” Take a quick photo of the used fluid if it looks dark or burned.
Finally, document your test drive, note the speed, gear, road type, temperature, and any tow weight in case you need to escalate.
Escalate with facts, not frustration
If the shop says they can’t reproduce the issue, request a ride-along and show the behavior at your speed and throttle input. Still stuck? File a formal complaint with NHTSA. One complaint may not move the needle, but a dozen identical ones might.
Quick contacts that actually matter
| Purpose | Where | What it gives you |
|---|---|---|
| Open recall check by VIN | NHTSA Recall Lookup | Shows unfinished safety recalls on your exact VIN. VIN Recall Lookup |
| GM recalls and programs | GM Owner Center | Recalls plus special coverages that may pay for hardware. Experience GM |
| Report a safety problem | NHTSA complaint portal | Creates a record that can push new investigations. NHTSA |
8. Straight answers to questions every Silverado owner asks
Why did my truck lock itself in 5th after the update?
That’s the TCM doing exactly what it was designed to do. It flagged valve wear approaching failure, so it capped the trans in 5th, flashed the MIL, and put up the “reduced propulsion” alert.
That move prevents a lock-up crash. Now it’s time to book service, reference N242454440, and get the valve replaced on GM’s dime.
Can a flush actually make things worse?
Yes, if it’s done wrong. High-pressure flushes can stir up sediment and jam solenoids. GM says no. TSB 18‑NA‑355 calls for a low-pressure, 20-quart exchange with Mobil 1 LV ATF HP. No shortcuts, no additive cocktails. Get that in writing on your RO.
My 2018 Silverado shudders, but I’m not in the recall. What gives?
That’s normal, unfortunately. The TCC shudder is covered by TSB 18‑NA‑355, not a recall, since it’s not classed as a safety risk. Some owners get relief with the fluid swap. Others chase it with torque converters when it comes back.
What if the dealer says, “We can’t reproduce it”?
Ask for a ride-along. Replicate it yourself, same route, speed, throttle. Take notes. Still getting brushed off? File a complaint with NHTSA. That paper trail is how new recalls are born.
Can I get reimbursed if I already paid to fix this?
Yes, if the repair lines up with the recall. GM offers reimbursement for qualifying out-of-pocket fixes done before the campaign launched. You’ll need your receipts. The dealer can walk you through the process, or you can submit to GM directly.
Silverado Transmission Trouble: What Matters, What Doesn’t
Recall 24V‑797 is the real deal. It targets 2020–2022 diesel Silverados and rolls out a software patch that blocks a dangerous downshift before it locks the rear wheels.
It won’t rebuild your transmission, but it gives early warning and unlocks a free hardware repair if wear shows up later. It’s not optional; get it handled.
Hydra‑Matic headaches? Whole different game. These 8-speed complaints live under TSBs, not federal recalls. Dealers follow GM’s fluid-and-software script, and results are all over the map.
Some trucks smooth out. Others keep shuddering until the torque converter’s replaced or a cooler gets added. Lawsuits are building, but in the shop, it still comes down to your VIN, your warranty, and how clearly you can describe the issue.
If your Silverado’s shifting rough, don’t wing it. Run the VIN. Close any open recalls. Match your symptom to the right bulletin. Keep your service records clean, and if you hit a wall, escalate with data. That sequence is how you stop chasing your tail and start steering this thing again.
Sources & References
- Chevy Silverado Recall List (2024 Update) – The Lemon Law Experts
- NHTSA Says GM Recalling 461K Vehicles Over Transmission Issues
- IMPORTANT SAFETY RECALL – nhtsa
- NHTSA Issues Safety Recalls for Multiple Vehicle Models Including Ford
- Common 8L90 Transmission Problems and How Upgrades Can Fix Them
- GM Fails To Get 8-Speed Trans Class Action Lawsuit Dismissed
- Service Bulletin TECHNICAL – nhtsa
- GM transmission class action to proceed – Class Action Lawsuits
- Safety Recall N242454440 Momentary Rear Wheel Lock-Up – nhtsa
- Class Action Lawsuit Against GM for Shuddering 8-Speed Auto Transmission Survives Motion to Dismiss – Corvette: Sales, News & Lifestyle
- Consumer Class Action Lawsuit Against GM 8-Speed Transmission
- GM Recall Information | GM account | GM
- Service Bulletin TECHNICAL – nhtsa
- My Son’s Silverado Was Having Transmission Issues And I Helped …
- DIY Transmission Flush : r/Silverado – Reddit
- Don’t Let High Temps KILL Your 10L80 Transmission or Valve Body – YouTube
- The GM 8-Speed TCC Shudder Solution – Lubegard
- Shudder fix? : r/Silverado – Reddit
- Bought a 2021 Silverado With 8-Speed Transmission and Told The Salesman That Gear Shifting Isn’t Smooth, After 30 Days I Took It To Dealership Again | Torque News
- Vehicle Safety Recalls Week – NHTSA
- Check for Recalls: Vehicle, Car Seat, Tire, Equipment | NHTSA
- NHTSA alerts motorist of vehicle safety recalls – WBIW
- GM’s Transmission DISASTER. They Won’t Fix It, So You Have To …
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Rami Hasan is the founder of CherishYourCar.com, where he combines his web publishing experience with a passion for the automotive world. He’s committed to creating clear, practical guides that help drivers take better care of their vehicles and get more out of every mile.