2016-2020 Cadillac CT6 Problems: 21 Known Issues

On certain 2016-2018 Cadillac CT6 vehicles, excess structural adhesive on the inboard child-seat anchorage bars can interfere with correct child-seat installation.

GM recalled 131 U.S. 2016 Cadillac CT6 sedans built on 21-22 July 2016 under NHTSA campaign 16V687 because the bolt that fastens the front passenger seat belt webbing to the anchor plate on the seat may have been omitted during assembly. Without that bolt the webbing can separate from its anchor, and the belt will not restrain the front passenger in a crash. GM told owners not to use the front passenger seat until the vehicle had been inspected — an unusually direct do-not-use instruction that marks this as a build-line escape rather than a wear item. It is narrowly date-bounded and easy to overlook when buying an early 2016 car.

GM recalled certain 2018-2019 Cadillac CT6 vehicles under NHTSA campaign 18V673 because second-row or third-row seat belt retractor assemblies may not switch into automatic locking mode when the belt is pulled fully out of the retractor. Automatic locking retraction is the mechanism that holds a child seat tight against the seat back; if it does not engage, the child seat can move freely on its belt and will not be properly restrained in a crash. Owners notice it directly: the belt pulls all the way out but never clicks into the ratcheting mode and simply retracts normally. Not every seating position on a given car is affected, so an owner may be able to move the child seat to a different position while awaiting the repair.

GM recalled 2020 Cadillac CT6 sedans fitted with dealer-installed accessory 20 x 8.5-inch wheels (RPO 5JO or SHH) under NHTSA campaign 19V731. The tire pressure monitoring sensors mounted in those accessory wheels transmit on the wrong radio frequency, so the car's TPMS receiver never hears them and the low-pressure telltale cannot illuminate when a tire goes soft. The vehicles fail to comply with FMVSS 138. Only 81 vehicles are affected, which is exactly why it is worth checking: a used 2020 CT6 wearing accessory 20s is the specific car at risk, and a driver can run a badly underinflated tire with no warning at all.

The CT6's rear camera mirror streams a live video feed from a roof-mounted rear camera onto the mirror glass. Owners report the feed going blank, showing a grainy, pixelated or heavily distorted picture, or losing focus so badly at night and in rain that the mirror is unusable. GM addressed the image-quality complaint in TSB 17-NA-375 for grainy, distorted or pixelated rear vision camera images on the CT6, but a share of cars have an outright module or camera failure that dealers resolved by replacing the assembly. Compounding it, using the rear washer sprays fluid onto the camera lens and smears it, and moisture accumulation defeats the lens's hydrophobic coating in wet, humid weather.

Certain 2019-2020 Cadillac CT6 vehicles equipped with a 10-speed transmission can develop internal transmission damage that may cause the wheels to lock while driving.

Powertrain complaints are the single largest NHTSA complaint category for the 2017 CT6. Owners of 2016-2018 cars report harsh or banging engagement between first and second gear, unpredictable shift quality, a long delay when moving from neutral or park into reverse, and occasional lurching or hesitation on tip-in. Some cars set transmission-controller codes. Unlike the TCC shudder, this is a shift-quality and control fault rather than a fluid-friction issue, and it has driven a meaningful share of CT6 lemon-law claims. On PHEV cars the same symptom set has a separate documented electrical root cause (see the transmission wiring-harness thermistor issue).

GM issued Service Update N202318000 in July 2021 for 2017-2018 Cadillac CT6 Plug-In Hybrid sedans, covering about 551 vehicles. The transmission wiring harness in these cars can have a defective crimp at the thermistor — the temperature-sensitive resistor that reports transmission fluid temperature to the controller. A bad crimp makes the temperature signal intermittent, and the four-mode hybrid transmission responds with erratic or intermittent shifting behavior. This is a PHEV-only campaign: the harness and the four-mode hybrid transmission are unique to the plug-in car and are not shared with the gasoline CT6's 8L45/8L90 or 10L80.

CT6 sedans with the 8L45 and 8L90 eight-speed automatic (2016-2018 model years) develop a torque converter clutch shudder that owners describe as driving over rumble strips — a vibration or shake at light throttle around 25-50 mph as the converter locks. GM traced it to the original factory-fill automatic transmission fluid being hygroscopic: absorbed moisture changes the fluid's friction characteristics and glazes the TCC friction material, producing slip-stick. This is a fluid-chemistry issue, not a worn converter, which is why replacing the converter without changing the fluid does not stick. It is distinct from the 10-speed transmission defect recalled separately on 2019-2020 cars.

GM issued Service Update 17408 covering 2016-2018 Cadillac CT6 sedans built with the 3.0L V6 twin-turbo (RPO LGW) after turbochargers were found capable of shedding turbine-wheel material. Owners report a whistle, siren or grinding noise from the turbo area followed by a hard loss of boost and a check-engine light. Because the LGW runs two small turbos, a single failed unit can drop the car into reduced power while the other side still spools, which is why the complaint is usually described as 'sudden loss of power under acceleration' rather than a no-start. Metal debris from a disintegrating turbine wheel can be drawn downstream, which is what turns a turbo replacement into an engine replacement in the worst cases. This is specific to the LGW twin-turbo — the 2.0L four and the naturally aspirated 3.6L LGX in the same body do not share it.

GM's February 2017 bulletin 17-NA-039 includes 2016–2017 CT6 vehicles with the LTG 2.0L turbo engine. It warns that certain engine oils can contribute to premature piston failure and specifies a low-calcium oil. The bulletin does not identify a diagnostic symptom pattern or fault-code list, attribute every piston failure to oil selection, or establish a warranty decision for an individual vehicle.

On certain 2019 Cadillac CT6 vehicles, steering-wheel rotation may not cancel the turn signal, which can mislead pedestrians and other drivers.

Certain 2016-2017 Cadillac CT6 vehicles have park and position lamps that may be excessively bright and fail FMVSS 108, potentially affecting other drivers’ vision.

GM's May 2021 revision of 17-NA-021 covers 2016–2018 CT6 vehicles with LGW or LGX engines. A hard brake pedal or air-type noise during application may be caused by oil in the booster. The symptom alone does not confirm contamination or justify replacing brake components.

On certain 2019 Cadillac CT6 vehicles, an electronic brake-control-module software error can disable ABS and stability control without illuminating the corresponding warning lamps.

GM opened a customer-satisfaction program in August 2016 for 2016 Cadillac CT6 sedans built with the 3.0L twin-turbo V6 (RPO LGW) and the KC4 cooling system. The engine oil cooler core can leak internally, pushing engine oil into the coolant circuit. Owners find a brown, sludgy emulsion in the coolant surge tank and, as the contaminated coolant loses heat-transfer capability and fouls the radiator and heater core, the engine can go into an over-temperature condition. The failure is tied to specific oil cooler manufacture dates rather than to every CT6 cooling system — the program instructed dealers to read the cooler's manufacture date before deciding to replace.

On some 2016–2019 CT6 vehicles with the 3.6L LGX or 3.0L LGW engine, water-pump noise can accompany DTC P0324. GM PIP5545C directs technicians to compare pump noise and knock-retard readings with the accessory belt installed and removed. If the tests do not support the pump as the source, follow normal service diagnostics.

CT6 owners report the radiator cooling fans running at high speed all the time — including after shutdown — with the coolant temperature gauge reading cold, and in many cases weak heater output and the A/C refusing to engage. The root cause is a thermostat that fails stuck open. The engine never reaches its target coolant temperature in the calibrated time, so the ECM sets P0128 (coolant temperature below thermostat regulating temperature), defaults the temperature gauge to the cold end, commands the A/C off, and commands the cooling fans on as a protective default. Owners frequently mistake this for a fan relay, a fan module or a bad temperature sensor and replace those first.

CT6 owners report the 12-volt battery going flat after the car sits for a few days, sometimes flat enough that the electronic shifter and door handles will not respond and the car needs a jump. GM documents a cycling battery draw on 2016-2020 CT6 vehicles in preliminary information bulletin PIC6359A: the keyless entry control module repeatedly wakes and polls for the key fob transmitters to support the approach-lighting feature, and the repeated wake cycles keep total draw above the acceptable sleep threshold. Other modules that fail to time out (infotainment, security) produce the same symptom, so the offending module has to be identified rather than assumed.

GM bulletin 17-NA-130 covers 2016–2017 CT6 vehicles equipped with Active Rear Steer (RPO NYS). A Service Soon message and U0435 may occur when the rear-steer module fails to initialize during remote start while the EBCM expects it to. This is a possible explanation for the described condition, not proof that every steering warning is software-related or that the steering hardware is fault-free.

A widely reported CT6 complaint is a creaking, ticking, popping or double-click noise from the roof and headliner area, heard during turns, over uneven pavement, and when the sunroof operates. GM documented it in TSB 16-NA-370 for 2016-2017 CT6 sedans equipped with the sunroof, attributing it to metal-to-aluminum contact between the rear glass trough and the guide rail, and metal-to-metal contact between the mechanism tab and the mechanism curve on the window slider. Because the CT6 uses an aluminum-intensive body, owners often assume the body structure itself is flexing; the documented source is the sunroof module hardware. Left alone it is a comfort defect rather than a safety one, but out-of-warranty sunroof assembly replacement has been quoted around $3,000.