The GM 8-speed shudder is one of those problems that feels way scarier than it is. The truck isn't falling apart. In most cases, the torque converter clutch is grabbing and slipping in quick little pulses because the original fluid wasn't up to the job. Fresh fluid of the right type fixes a lot of them.
Which Sierras this applies to
GM's TSB 18-NA-355 covers the 8L90 and 8L45 8-speed automatics (RPO codes M5N, M5T, M5U and M5X). On the GMC side it lists the 2015-2018 Sierra 1500, along with the 2015-2017 Yukon and 2017-2019 Canyon. The same bulletin also lists the 2015-2018 Chevy Silverado 1500, 2017-2019 Colorado and 2015-2017 Cadillac Escalade, plus several Corvettes, Camaros and Cadillac cars.
Not every 2015-2018 Sierra has the 8-speed. Earlier and lower-trim trucks may use the 6-speed. Your RPO codes are on the sticker in the glovebox, so look for M5N, M5T, M5U or M5X.
What it feels like
- Torque converter shudder: a vibration or shake during light, steady acceleration. It feels like rumble strips, or like driving on a washboard road for a second.
- Harsh shifts: clunky 1-2 upshifts and 2-1 downshifts, mostly at light and medium throttle.
GM Authority reported that GM traced the issue to moisture in the original transmission fluid. On July 8, 2019, GM switched to the new Mobil 1 fluid at the factory, and starting August 7, 2019, dealers were told to flush affected new trucks still sitting on their lots.
What GM tells dealers to do
| Step | What happens |
|---|---|
| 1 | Confirm the shudder on a test drive and check for codes |
| 2 | Exchange the old fluid with Mobil 1 Synthetic LV ATF HP (a full exchange, not just a pan drain) |
| 3 | Drive normally for about a week so the shift adapts relearn |
| 4 | Shudder still there? Torque converter replacement is the usual next step |
What it costs
| Situation | Who does it | Likely cost |
|---|---|---|
| Truck still under GM powertrain warranty | GMC dealer | Usually $0, ask them to apply TSB 18-NA-355 |
| Fluid change, 2011-2018 Sierra 1500 | Independent or dealer | RepairPal: $387-$501 |
| Fluid change, 2019-2021 Sierra 1500 | Independent or dealer | RepairPal: $620-$859 |
| Torque converter replacement | Dealer or transmission shop | Get a written quote, the transmission has to come out |
Easy things to try first
- Check your service history. If a dealer already did the 18-NA-355 fluid exchange, it should be in the GM records tied to your VIN. Any GMC dealer can look it up.
- Note when it happens. Speed, throttle and temperature. A shop can reproduce it much faster if you tell them "light throttle around 40 mph, warmed up."
- Rule out tires and driveline. A vibration that's there at all throttle positions and gets worse with speed is more likely a tire balance or driveshaft issue, not torque converter shudder.
- Give it a week after any fluid service. The transmission needs time to relearn.
When to call a pro
If the shudder is getting worse, the check engine light comes on, or you notice slipping or flaring between gears, don't keep driving it for months. A glazed torque converter clutch can shed debris into the fluid. GM Authority notes that if the flush doesn't fix it, the converter itself may be glazed and need replacement.
What to ask before you pay
- "Has TSB 18-NA-355 been done on this VIN?"
- "Is this a full fluid exchange with Mobil 1 LV ATF HP?"
- "Is the transmission still under my powertrain warranty?"
- "If the fluid doesn't fix it, what's the torque converter quote, and does that include new fluid again?"
Is it worth fixing?
Yes. A fluid exchange is cheap compared to any internal repair, and it fixes a lot of these trucks. Even the torque converter step is far cheaper than a whole transmission. Don't let anyone sell you a rebuild for a shudder-only complaint until the fluid exchange has been done and given time to relearn.
More Sierra help lives on the GMC Sierra 1500 page. Got a warm-air AC problem on a 2019-2021 truck? See our Sierra AC condenser guide. Prices checked October 14, 2026.
