OBD-II trouble code
P0829: 5-6 Shift
Not a switch code and not a circuit code — a mechanical shift failure sitting in the middle of a run of electrical ones. It names the last upshift in the box, the one that only happens at sustained cruise, which is why most owners get the light before they ever get a symptom.
Quick facts
- System
- Powertrain
- Category
- Transmission / Shift Control
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $180 – $6,500
- DIY difficulty
- Shop recommended
What does P0829 mean?
P0829 says the transmission was commanded to change from fifth to sixth and the change either did not happen, did not finish, or did not hold. The controller knows because it watches input and output speed: for every gear there is one ratio between those two numbers that is correct, and after a shift command the measured ratio has a short window to arrive at the new value and stay there. When it does not, the code is stored against the specific pair of gears involved.
What makes this particular pair worth its own page is where it sits in the box. Fifth to sixth is the last upshift a six-speed can make and the top of the overdrive range, which means it happens under a narrow set of conditions and almost nowhere else: light throttle, a warm transmission and a road speed the vehicle can hold. That is a highway, and only a highway. Around town the shift is never commanded at all. Owners therefore find this code after a long trip and can drive for weeks afterwards without meeting it again, which makes it one of the easier codes to talk yourself out of.
The second consequence of its position is that the ratio step from fifth to sixth is the smallest in the gearbox. Lower gears are spaced widely because they have to cover a lot of ground; the top two are close together because they are both doing the same job at different degrees. A slipping or incomplete shift between two closely spaced ratios produces very little of the flare that a bad one-two shift makes obvious. There is no lurch, no bang, no rise in engine speed you would notice with the radio on. What there is instead is a small, steady loss of fuel economy on the motorway and an engine sitting a couple of hundred revolutions higher than it used to at the same speed — the kind of change that gets attributed to weather, tyres or fuel long before anyone suspects the transmission.
That combination is exactly why the code deserves to be taken seriously despite the mild symptoms. On a modern clutch-to-clutch automatic the shift is achieved by releasing one clutch pack and applying another with overlapping timing. If the apply is late, weak or short of pressure, the two elements spend time slipping against each other, and slipping generates heat in fluid that is already working near the top of its temperature range at motorway speed. Heat is what degrades the fluid, and degraded fluid is what makes the next shift worse. The failure is progressive and it accelerates. Fluid level and condition are the first things to check for that reason, not because low fluid is the most interesting answer but because it is both the most common cause and the one that gets cheaper the sooner you deal with it.
Common causes
- Low transmission fluid level, or fluid degraded by heat and mileage
- Failing 5-6 shift solenoid, sticking or slow to respond to command
- Clogged or restricted valve body passage limiting apply pressure to the sixth-gear element
- Worn clutch pack or band for the gear being applied, allowing slip during the shift
- Internal leak past a worn seal or sealing ring, so commanded pressure never reaches the element
- Failing pressure control solenoid producing inaccurate line pressure at low throttle
- Input or output speed sensor reporting inaccurately, so the ratio check fails on bad data
- Wiring or connector fault at the transmission pass-through connector affecting solenoid control
- Valve body wear allowing pressure to bleed off, common at high mileage on some designs
- Transmission control module software calibration out of date on vehicles with a known update
Symptoms
- Check engine light after a long motorway run, often with no complaint at all
- Engine turning a few hundred rpm higher than usual at a settled cruise
- Measurable drop in highway fuel economy with town economy unchanged
- Transmission holding fifth and refusing to take top gear
- Brief slip or flare felt only at light throttle at high speed
- Transmission dropping into a limited operating mode on long climbs
- Symptoms absent entirely in city driving and present only on the open road
- Transmission fluid smelling burnt or reading dark on vehicles with a dipstick
Diagnostic steps
- 1.Check fluid level and condition before anything else, using the manufacturer's procedure and temperature. This is the single most common cause and the only one that gets more expensive the longer it waits.
- 2.Read every stored code, not just this one. A 5-6 shift fault sitting alone points at that circuit; one arriving with pressure or ratio codes for other gears points at a shared hydraulic problem instead.
- 3.Record live data on a motorway run rather than in the workshop, since the shift is only commanded at sustained cruise. Log input speed, output speed, gear commanded, gear achieved and fluid temperature together.
- 4.Compare commanded gear against calculated ratio through the shift. A commanded sixth that never reaches the sixth-gear ratio is a genuine mechanical or hydraulic failure; a correct ratio with the code still setting suggests a speed sensor reporting badly.
- 5.Watch fluid temperature across the same drive. A transmission running hotter than normal at cruise supports slip, and gives you a number to compare against after the repair.
- 6.Perform a line pressure test where the transmission provides a port for it. Pressure low across the board points at the pump or a pressure control solenoid rather than at the 5-6 element specifically.
- 7.Check for an applicable software update or recalibration before opening anything. Some designs have known shift calibration revisions and the physical transmission is fine.
- 8.Inspect the external harness and the pass-through connector for the solenoid pack. Fluid wicking up a wire is a real failure route and looks nothing like a mechanical problem from the driver's seat.
- 9.Only then consider internal inspection, and get a scan of the valve body and solenoid resistances first. The difference between a solenoid and a clutch pack is the difference between a modest bill and a large one, and it is worth measuring rather than guessing.
Repair cost
$180 – $6,500
The range is wide because the causes genuinely span that far. A fluid and filter service, which fixes a real proportion of these when caught early, is $180 to $450. A software recalibration where one applies is $90 to $200. Solenoid replacement is $50 to $300 in parts plus 2 to 5 hours, so $350 to $1,100 depending on whether the pan or the whole valve body has to come out. A valve body replacement runs $500 to $1,600. If the sixth-gear clutch pack has been slipping long enough to burn, a rebuild or a reman unit is $1,200 to $5,500 in parts with 6 to 14 hours of labour, putting that outcome at $2,000 to $6,500. The gap between the first line and the last is the argument for not driving on it for a season.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with transmission fluid service preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.