Why does my Cadillac CT4 transmission shudder or not shift right?

Short answer: A shudder at steady speed on a CT4 with the 8-speed is usually torque converter shudder, and GM's fix is a flush with updated fluid (TSB 18-NA-355). If it won't shift or sets check engine lights, get the codes read. If you have a 2020-2021 CT4 with the 10-speed, check recall 25V148000.

Symptom: A vibration like rumble strips at 25-80 mph on light throttle, harsh or delayed shifts, the car stuck in first, or a check engine light.

What to check first

1. Note when it shudders: steady speed, light throttle, and what speed.
2. Get the codes read.
3. Ask the dealer about TSB 18-NA-355 and the approved fluid.
4. Check whether your CT4 has the 8-speed or the 10-speed.
5. Run your VIN at nhtsa.gov/recalls for 25V148000.

Likely causes

1. Torque converter shudder (8-speed). The torque converter connects the engine to the transmission. On GM's 8-speed, owners said it shudders at steady light throttle, like driving over rumble strips. GM bulletin 18-NA-355 calls for a flush with updated fluid.
2. Transmission fault needing repair. One CT4 owner's car wouldn't shift out of first and set check engine lights. It went to the dealer under warranty.
3. 10-speed internal damage (recall). Recall 25V148000 covers certain 2020-2021 CT4s with the 10-speed: the transmission can be damaged internally and lock the wheels.

What it usually costs

Owners had the flush and repairs done under warranty. One said a scheduled maintenance flush fixed it at 105,000 miles. They didn't post out-of-warranty prices.

Is it safe to keep driving?

A mild shudder isn't an emergency, but get it fixed. If it won't shift, slips or makes grinding noises, stop driving it. The 10-speed recall says the wheels can lock up.

DIY or shop?

No. Use the exact fluid and procedure in the bulletin, which takes a dealer or a well-equipped shop.

Recalls, bulletins and complaints in plain words

• Recalls: NHTSA lists recall 25V148000 for certain 2020-2021 CT4 (and other GM models) with the 10-speed transmission: it can be damaged internally and cause the wheels to lock up. If your car is part of this recall, the dealer fixes it for free. Check your VIN at nhtsa.gov/recalls.
• Plain words: The complaint count below shows how often owners reported problems that mention these words to NHTSA. A count is not proof of a defect. NHTSA's recall lookup returned no data for 2024-2025.
• NHTSA complaints: 10 keyword-matched complaints for Cadillac CT4 2020–2025 (of 33 unique complaints queried, NHTSA complaints API by model year, queried 2026-11-04; keywords: shudder, transmission, shift, jerk, harsh, torque converter, gear) (NHTSA returned no data for 2 model-year queries). This counts complaints that mention these words. It is not a defect finding.

What owners report

• In 2021 CT4 8 Speed Auto Help on Reddit r/Cadillac, a 2021 CT4 2.0 with 36k miles had two check engine lights in a week. The torque converter didn't seem to engage and it wouldn't shift out of first. It was a CPO car, so the dealer fixed it under warranty.
• In AT8 shudder - can it be solved? on Reddit r/Cadillac, owners of Cadillacs with GM's 8-speed said the shudder is common. Several had it fixed with a dealer fluid flush under TSB 18-NA-355 using the approved fluid, including an ATS owner and a 2017 Escalade owner. One said it was gone 10k miles later, and another's started at 105,000 miles and was fixed with a flush.

Quick tip

Ask for the bulletin flush with the approved fluid first. It fixes most shudder complaints.

Vehicle focus: Cadillac CT4, 2020-2025 (8-speed and 10-speed automatics).

Safety / mechanic note

Wheel lock-up or a sudden loss of drive is dangerous. Verify with a mechanic.