OBD-II trouble code
U0402: Invalid Data Received from Transmission Control Module
The transmission controller is still talking but what it reports has been rejected as impossible. Before any of this is treated as an electrical problem, check the fluid — a transmission low on fluid genuinely does behave in ways its own controller cannot describe sensibly.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $120 – $2,500
- DIY difficulty
- Shop recommended
What does U0402 mean?
The transmission controller is unusual among the modules on a vehicle network, because most of what it reports is not electrical at all. It reports hydraulics: line pressure, clutch apply and release, the ratio between input and output shaft speeds, and the temperature of the fluid that makes all of it work. Those numbers describe a mechanical system in a state of flux, which means there is a possibility here that does not exist for most invalid-data codes — that the data really is impossible, and the reason is that the transmission really is doing something impossible.
That is why fluid comes first. A transmission low on fluid, or with fluid that has been aerated by a whipping pump pickup, will slip. Slip means the input shaft is turning faster than the output shaft can account for in any gear the controller thinks it has selected, so the calculated ratio is a number that does not correspond to any gear. Pressure will also read wrong, because air compresses and fluid does not. The controller broadcasts those values honestly, other modules reject them as implausible, and this code appears. Checking level and condition on the correct procedure for the vehicle — many require it hot, in gear, with the engine running — takes minutes, costs nothing, and settles whether this is an electrical investigation at all.
Temperature is the second thing that makes this code behave unlike its siblings. Automatic transmission fluid changes viscosity dramatically between a cold morning and a fully warmed system, and the controller's expectations shift with it. A fault that only sets in the first few minutes of a cold start, or one that only appears after half an hour of motorway driving, is telling you something specific: cold-only points at fluid that has thickened past what a marginal pump or a partly blocked filter can move, while hot-only points at fluid that has thinned to where a worn clutch or a leaking seal finally lets go, or at a solenoid whose winding resistance has climbed with heat. That temperature signature is worth capturing from the freeze frame before anything is dismantled, because it narrows the search more than any single measurement.
The third fact is about where the controller physically lives, and it is the one that explains a large share of genuine electrical faults on this code. On a great many modern vehicles the transmission controller is not in the cabin or the engine bay — it is bolted inside the transmission itself, submerged in fluid, on a valve body that heats past a hundred degrees Celsius and cools again every day of its life. Its wiring runs through an internal harness to a pass-through connector moulded into the case. That arrangement produces failures with a signature of their own: the case connector is a known leak and corrosion point, fluid can wick along the internal harness and eventually reach the module, and years of thermal cycling harden internal insulation until a conductor breaks. None of that is visible from outside, and none of it is found by inspecting engine bay wiring. If a vehicle has an internal controller and there is any sign of fluid seepage at the case connector, that finding outranks everything else on the list.
There is also a purely software cause that is specific to this module. Transmission controllers learn. They hold adaptive values for how long each clutch takes to fill and apply, and they refine those values over thousands of shifts to keep the changes smooth as components wear. Replace the controller, disconnect the battery on some designs, or fit a rebuilt unit, and those adapts return to default — and a controller running default adapts on a transmission that has covered a hundred thousand miles can produce shift behaviour and reported values that other modules find implausible. The fix is a relearn procedure rather than a part, and skipping it after a repair is a common reason this code returns immediately.
As for consequences, this code has a more concrete one than most. When the transmission controller cannot be trusted, the usual response is a fail-safe that locks the transmission into a single fixed gear — commonly third or fourth on a rear-drive automatic — so that the vehicle can still be driven at moderate speed to somewhere it can be repaired. That is a designed behaviour, not a breakdown, and it is useful to recognise: it means limited acceleration, no shifting, and often no reverse on some designs, but it also means the vehicle is generally driveable in the short term. Whether it should be driven depends on the fluid check, because driving a slipping transmission is what turns a fluid problem into a rebuild.
Common causes
- Low or aerated transmission fluid causing genuine slip, so the reported gear ratio corresponds to no real gear
- Fluid degraded, burnt or overdue, changing the pressures and fill times the controller reports
- Fluid seepage or corrosion at the transmission case pass-through connector, where an internally mounted controller wires out
- Internal transmission harness damaged by heat cycling or fluid wicking along the conductors
- Failing input or output shaft speed sensor, producing a ratio calculation the controller broadcasts as impossible
- Fluid temperature sensor reading incorrectly, which shifts every expectation the controller works from
- Shift or pressure control solenoid failing, particularly one whose resistance climbs as the fluid heats
- Adaptive shift values reset to default after a controller replacement or battery disconnection, with no relearn carried out
- Transmission controller replaced, rebuilt or fitted second-hand without being programmed for this vehicle
- Weak battery or failing charging system producing marginal voltage and implausible reported values
- Internal controller fault, which belongs after fluid, wiring and programming have been cleared
Symptoms
- Transmission locked in a single fixed gear, with no shifting and limited acceleration — the designed fail-safe response
- Harsh, delayed or flaring shifts, or a shift that never completes
- Engine revving without a corresponding increase in road speed, which is slip and points at fluid before electronics
- A fault with a clear temperature signature — only when cold, or only once thoroughly warm
- Fluid visible at the transmission case connector or a fluid smell after a run
- Speedometer or gear display misbehaving on vehicles that take those values from the transmission controller
- Cruise control and stability functions unavailable because they cannot trust the transmission data
- Symptoms starting immediately after a transmission repair, controller replacement or battery disconnection
Diagnostic steps
- 1.Check transmission fluid level and condition using the vehicle's correct procedure, which is frequently hot, in gear and with the engine running. Slip caused by low or aerated fluid produces genuinely impossible reported values and costs nothing to rule out.
- 2.Note the temperature at which the fault occurs from the freeze frame. Cold-only and hot-only faults point at different mechanisms and narrow the search substantially.
- 3.Inspect the transmission case pass-through connector for fluid seepage and corrosion where the controller is mounted internally. Fluid reaching an internal harness is a leading cause and is invisible from the engine bay.
- 4.Read the full code set. Transmission ratio, solenoid and speed sensor codes stored alongside this one usually name the actual fault directly.
- 5.Compare input and output shaft speed readings in live data against the gear the controller believes is engaged. A ratio that matches no gear is mechanical slip, not a communication problem.
- 6.Check the fluid temperature reading against reality. Every pressure and timing expectation the controller holds is scaled by that value, so a wrong temperature makes everything else look wrong.
- 7.Test the battery and charging system under load, since marginal voltage produces implausible values without any component being faulty.
- 8.Establish whether the controller has been replaced, rebuilt or reprogrammed recently, and whether the adaptive relearn procedure was carried out. Default adapts on a worn transmission produce exactly this complaint.
- 9.Verify the controller carries the correct calibration for this vehicle and check for outstanding software updates addressing known shift-quality faults.
- 10.Consider replacing the controller only once fluid, internal wiring, sensors, voltage and programming have all been cleared, and plan for the relearn that will follow.
Repair cost
$120 – $2,500
The range is wide because this code can be a fluid problem, a wiring problem or a transmission problem. A fluid service to correct level and condition is $120 to $350 and is where a meaningful share of these end. Repairing or replacing the case pass-through connector and internal harness is $250 to $900, most of it access. A speed sensor or fluid temperature sensor is $150 to $450 fitted. A solenoid or solenoid pack is $300 to $1,000. Programming or carrying out an adaptive relearn is $100 to $300 and sometimes resolves the whole complaint after other work. Replacing the transmission controller runs $500 to $1,300 with programming. If the fluid check reveals genuine slip from internal wear, that is a transmission repair rather than a module repair and is priced separately and considerably higher.
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.