What's Actually Going Wrong With That Six-Speed
The 2017 Buick LaCrosse uses GM's 6T45 six-speed automatic transmission. It's the same unit found across a wide range of General Motors vehicles from that era, and it carries the same set of predictable failure modes. Most owners don't realize they have a problem until the symptoms become loud enough to ignore. The transmission control module is usually the first place to look, but it's not always the root cause. I spent three days last month diagnosing a 2017 LaCrosse that had come in with a complaint about rough shifts and a confirmed P0846 code. The OBD-II scanner was reading "Transmission Fluid Pressure Sensor B Intermittent." Most shops would swap the sensor and send it back out. The actual issue was a corroded pin in the connector housing at the transmission wiring harness. The connector on the driver's side, near the firewall, had accumulated road grime and moisture over the years. I cleaned the pins with electrical contact cleaner, applied dielectric grease, and reinstalled the connector. The code cleared after a drive cycle and hasn't returned. That's the kind of thing that saves you from replacing a perfectly good sensor.
Shifting Gears Uncovering The Common Transmission Issues In The 2017 Buick Lacrosse
Common Failure Points
Shift Quality Deterioration
The most frequently reported issue is harsh or delayed shifting, particularly between first and second gear and occasionally during the third-to-fourth transition. This isn't always a mechanical failure inside the valve body. A significant number of cases trace back to degraded transmission fluid and worn shift solenoids. The 6T45 requires Dexron VI fluid specifically. Using anything else or topping off with synthetic blends that don't meet that specification will alter the friction characteristics and throw off the shift timing calibration. The TCM learns shift adaptions over time based on fluid behavior, and wrong fluid means wrong adaptations. When the adaptive values drift too far, the TCM can't compensate and you get the clunk or the flare that people notice on the highway. Another factor is the main pressure regulator valve in the valve body. Over higher mileage, typically above 80,000 miles, wear on this valve can cause erratic line pressure. The symptom is a shudder or a firm thud during acceleration shifts that seems random. Replacing the valve body assembly is one approach, but a lot of the time an incomplete rebuild of the existing valve body with new regulator sleeves fixes it at a fraction of the cost. I've seen shops charge $1,800 for a new valve body when a proper machine shop rebuild with new bores costs closer to $600 and lasts just as long.
Solenoid Failures
The 6T45 has multiple solenoid packs: the line pressure solenoid, the shift solenoids, and the torque converter clutch (TCC) apply solenoid. These are the components most likely to cause trouble signals. The TCC solenoid is especially problematic. When it starts to stick, you'll feel a shudder around 40 to 50 mph under light throttle load. This is different from a tire balance issue or a bad motor mount, which can feel similar but occur at different speeds and under different conditions. The shudder from a sticking TCC solenoid comes and goes with throttle input. Ease off the gas slightly and it usually fades. Solving this isn't always straightforward. Some people reach for chemical flushes or aftermarket additives, and those will temporarily mask the symptom by changing the fluid viscosity enough to free a stuck solenoid. But the fix doesn't last. The real solution is replacing the faulty solenoid and doing a proper fluid exchange. I recommend using a multi-stage flush that pulls old fluid from the pan and replaces it in stages rather than a single high-pressure flush that can dislodge debris and push it into the valve body passages where it causes more problems downstream.
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Electronic Control Module Issues
The TCM itself can develop internal faults. Corrosion on the TCM connectors is a known issue, particularly on vehicles that see winter driving conditions. Salt and road moisture get into the connector housings and eat away at the contacts. This causes intermittent communication errors between the TCM and other modules on the CAN bus. The dashboard will throw a check engine light and sometimes a transmission warning message. Resetting the code might clear it temporarily, but it will come back because the underlying connection is still degraded. I learned this the hard way on a vehicle that came in with a no-start complaint that turned out to be transmission-related. The TCM wasn't communicating with the scan tool. We traced it to a corroded ground strap under the vehicle near the transmission tunnel. The ground point had been painted over during a previous body repair and lost its bonding. Cleaning the paint off the ground surface and reinstalling the strap with a proper star washer fixed the communication issue immediately. Nobody checks ground straps unless they already suspect an electrical problem, so this kind of diagnosis takes time and a willingness to follow the wiring diagram rather than the code.
When to Replace Rather Than Repair
There's a threshold where spending money on individual components stops making financial sense. If the transmission has over 120,000 miles and you're seeing multiple failure modes simultaneously, a complete rebuild is usually more reliable than addressing each issue as it comes up. The internal clutch packs, the drum seals, and the planetary gear sets all experience wear at roughly the same rate. Replacing just the solenoids or the valve body leaves the worn clutches and drums in place, and they'll be the next thing to fail. A quality rebuild on the 6T45 typically runs between $2,200 and $3,000 depending on the shop and whether you need a remanufactured or new case. A used transmission from a salvage yard might be listed for $800 to $1,500, but the mileage on the donor vehicle matters enormously. A 90,000-mile donor with no service history is a gamble. A 50,000-mile donor with documented fluid changes is a much better bet if you're working with a tight budget. Either way, expect to pay another $400 to $600 for labor on top of the parts cost.
Maintenance Practices That Actually Matter
Fluid changes are the single most effective preventative measure. The manufacturer's official interval on the 2017 LaCrosse is listed as lifetime fill, which means it won't be changed under normal warranty service. "Lifetime" in GM terms from this era generally corresponds to the expected useful life of the vehicle's powertrain under average driving conditions, which is somewhere between 100,000 and 150,000 miles for most owners. If you plan to keep the car beyond that, changing the fluid every 40,000 to 50,000 miles is reasonable. This includes replacing the filter screen inside the pan as well. The pan bolts are 10mm, which means some shops will try to reuse them and strip them. Bring your own set or budget for replacements. The filter itself is a simple drop-in part and costs around $15 to $25. The fluid capacity is approximately 7.4 quarts for a complete dry fill, though a drain and fill from the pan typically returns about 3.5 to 4 quarts. I always recommend a two-stage fluid exchange to maximize the amount of old fluid displaced. After the first drain and fill, run the engine through all gears and then check the fluid level with the dipstick while the transmission is at operating temperature. Top off as needed and take it for a drive. Check again the next day. The adaptive learning will settle in after a few hundred miles of normal driving, and if you've been replacing worn components, the shift quality should improve noticeably within that distance.
Red Flags That Require Immediate Attention
A burning smell coming from the transmission area is never normal. It indicates either severely degraded fluid or internal slippage causing excessive heat. If you notice the smell along with a loss of power or a hesitation during acceleration, stop driving the vehicle and arrange for a tow. Continuing to operate a slipping transmission will destroy the clutch plates and possibly damage the drum internals, turning a solenoid replacement into a full rebuild. Slipping is usually confirmed by watching the RPM gauge during acceleration. If the engine revs increase without a corresponding increase in vehicle speed, the transmission is slipping. Another warning sign is the transmission going into limp mode. You'll notice the vehicle locks into a single gear, usually third or fourth, and you lose access to the other gear ranges. This is the TCM's protection strategy when it detects an irreversible fault. Limp mode is not a temporary condition you can drive around for a while. It requires diagnostic attention before any further operation. A proper scan with a GM-compatible tool will pull the specific code and data stream information that generic OBD-II readers often miss. The difference between a $100 code reader and a professional diagnostic tool is exactly this kind of detail.
Parts Sourcing Considerations
GM transmission components are widely available, but the quality varies significantly between OEM and aftermarket parts. The solenoid assemblies are one area where aftermarket versions are noticeably inferior. The internal tolerances on reproduction solenoids are looser, and the flow characteristics differ from the originals. This causes the TCM's adaptive values to drift out of spec again within a relatively short time. I always recommend genuine GM or at minimum ACDelco Professional-grade solenoids. The price difference is maybe $10 to $20 per solenoid, and it's worth it. Valve body gaskets and seals are similarly sensitive. The material composition matters for maintaining proper fluid pressure. Cheap rubber alternatives swell or harden prematurely, leading to the same pressure-related symptoms that brought you in the first place. Again, stick with OEM or OEM-equivalent seals. They cost marginally more but they last the intended lifespan of the repair.
What Independent Shops Get Wrong Most Often
The most common mistake I see is misdiagnosing a transmission issue as a mechanical problem when it's actually electrical. A rough shift code might point to a specific solenoid, and the shop replaces it without checking the wiring harness, the connector, or the power and ground circuits to the TCM. The solenoid tests fine in isolation. It's the wiring that's intermittent. The customer comes back two weeks later with the same complaint and a new code, and now there's distrust on both sides. The second common error is over-recommending transmission replacements. A lot of the symptoms that look like internal mechanical failure are actually simple electrical or fluid issues. The shudder, the harsh shifts, the limp mode warnings, the codes. All of these have multiple possible causes, and the cheapest causes should always be investigated first before moving to the expensive ones. If a shop is quoting a full transmission replacement as the first option without ruling out solenoids, wiring, or fluid issues, you should get a second opinion. It usually takes less than two hours of proper diagnostics to identify the actual root cause, and that hour of labor is far less expensive than a rebuilt transmission you didn't need. I've been working on these transmissions for long enough to recognize the patterns, and I still find myself pulling wiring diagrams and checking continuity rather than jumping to conclusions. That habit has saved more owners money than I can count.
