U0402
NetworkInvalid Data Received From Transmission Control Module (TCM)
The engine ECU is constantly chatting to the gearbox computer (the TCM) over the car's internal data network. When the messages it gets back are scrambled, out of range, or just don't make sense, it logs U0402. For you as the owner, that almost always points to a communication or wiring problem between the two modules rather than a knackered gearbox. The transmission itself is usually mechanically fine when this code shows up.
ⓘ Information only. This page provides general educational information about fault code U0402. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does U0402 mean?
U0402 is a Network (CAN bus, module communications) fault code. It indicates: Invalid Data Received From Transmission Control Module (TCM).
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine or gearbox warning light on the dash, sometimes both
- • Car drops into limp mode and sticks in one gear, often third, with restricted power
- • Harsh, jerky or badly timed gear changes that feel nothing like normal
- • Long delay or no response when you select Drive or Reverse
- • Traction control or stability lights flickering on and off alongside the main warning
- • Hesitation or a kick during acceleration
Possible causes
Causes commonly associated with U0402, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Corroded or water-damaged wiring and connectors on the CAN bus harness. This is the bread and butter cause, especially on cars that have sat outside or had a leak near a module connector
- 2. Weak battery or a poor earth causing voltage to sag. Modules get confused and start sending garbage data when the supply isn't stable, so check this before condemning anything expensive
- 3. Damaged or chafed wiring in the transmission loom, particularly the runs under the car that take road salt and stone chips
- 4. Corrupted TCM software or a firmware update that didn't complete properly, common after a botched dealer reflash or a battery going flat mid-update
- 5. Internal failure of the TCM itself, often from moisture getting into the unit on transmissions where the controller sits in or near the gearbox
- 6. CAN bus termination resistor faulty or a loose connection somewhere on the network
- 7. Faulty engine ECU sending or interpreting messages wrongly. Less common, but it does happen
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Scan the whole car, not just the engine. U0402 rarely shows up alone. Note every U-code and powertrain code, because a string of communication faults usually means a wiring or network problem rather than a single dead module
- 2. Test the battery and check every earth strap and battery terminal for corrosion or looseness. A tired battery or dodgy earth throws up communication codes that vanish once the supply is sorted
- 3. Get under the car and inspect the gearbox harness and connectors. Look for green corrosion, water in the plugs, chafed insulation, and pushed-back pins. Wiggle the connectors with the ignition on and watch for the fault flickering
- 4. Backprobe the CAN-H and CAN-L lines with a multimeter. You're looking for roughly 2.5V on each at idle and around 60 ohms across the pair with the system powered down. Wild readings point to a broken line or a failed resistor
- 5. Use a scan tool to command the TCM directly with a bidirectional test and watch whether it responds. No response or junk data after the wiring checks out narrows it to the module or its software
- 6. Check the manufacturer's technical service bulletins. A fair number of these clear with a software update, so it's worth knowing before anyone orders a new module
Common questions about U0402
Can I sort this myself or do I need a garage? +
You can do the easy half yourself. Cleaning up battery terminals, checking earth straps, and inspecting the gearbox connectors for water and corrosion costs nothing and fixes a decent share of these. Cleaning a corroded plug and packing it with dielectric grease has cured plenty of U0402s. Beyond that you're stuck, because confirming a CAN bus fault and reprogramming or coding a TCM needs a proper diagnostic tool and the carmaker's software. If the wiring and battery are clean, hand it to a garage with the right kit rather than throwing parts at it.
If I clear the code, will it stay gone? +
Depends entirely on what caused it. A one-off voltage glitch from a flat battery might clear and never return once the battery's charged. But if there's corroded wiring, a dying TCM, or a software fault, the code comes straight back, usually within a few miles or the next time the car gets damp. Clearing it is fine as a test to see if the fault is live or historic. It is not a fix. If the warning light returns, you've got an ongoing fault that needs the actual cause found.
What happens if I just keep driving with it? +
Two problems. First, the car will likely stay in limp mode, so you're stuck with one gear and weak performance, which is miserable and a bit risky on a motorway slip road or pulling out at a junction. Second, if the same communication fault is knocking out traction control or stability systems, those safety nets may not work when you need them. The gearbox usually isn't being damaged mechanically, so it's not a grenade waiting to go off, but driving a car that randomly loses drive or changes gear badly is asking for trouble.
How quickly do I need to deal with this? +
Soon, but you don't need to panic. If it's in limp mode you'll know about it immediately and most people get it looked at within a few days because the car's barely driveable. If it comes and goes and the car still moves normally, you've got a bit more breathing room, though I'd still book it in within a week or two before the fault gets worse and strands you somewhere. Repair costs swing wildly: a wiring repair or software reflash often lands in the low to mid hundreds, while a replacement TCM with programming at a main dealer can climb past a grand. An independent gearbox specialist will usually undercut the franchised dealer.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →