U0549: Invalid Data Received From Remote Accessory Module
What to do
The car drives normally. This covers things done remotely — remote start, preconditioning, app control.
The first question is whether anything has been added to the car. Remote start kits, insurance black boxes, trackers and app-based systems all live here. If one has been fitted, go back to whoever installed it — badly spliced connections and devices that keep the car awake are the two common faults, and they are the installer's to fix.
The symptom that costs money is a flat battery after the car stands for a few days. A device that never lets the car go to sleep is the usual cause, and it is worth mentioning specifically.
Do not simply unplug something to make the code go away. This function can request doors and ignition, and a half-disconnected installation is a security problem as well as an electrical one — have it removed or repaired properly.
What this code means
The remote accessory module is broadcasting data that cannot be accepted as true. See U0400 for what an invalid-data code is and why the module it names is usually the messenger rather than the culprit.
Before anything is diagnosed, establish that the vehicle actually has this equipment.
This is optional kit, and the commonest cause of a code naming it is a car that was told it has one when it does not. A body control module or gateway coded to a higher specification, a second-hand module carrying its donor vehicle's configuration, or a retrofit that was wired in but never coded — all three report equipment that is not there, and none of them is a fault in the vehicle.
Nothing is broken in that case and nothing needs replacing. The repair is a coding correction, and it is quick when somebody asks the question first.
"Remote accessory" covers the things operated from outside the car or before you reach it — remote start where it is offered, remote climate preconditioning, remote locking of ancillary equipment, and on many vehicles the interface a telematics or app service uses to reach the vehicle.
Which makes the aftermarket the leading suspect by a wide margin. Remote start kits, insurance telematics boxes, fleet trackers and app-based immobilisers all attach here or emulate this function. A module reporting implausible data on a car that has had any such device fitted is usually reporting the device, and the installer is the right person to ask.
Two things characterise a poorly-installed one. Splices into factory circuits that back-feed or load them unexpectedly, and a device that keeps the network awake so the vehicle never sleeps. The second is the one owners notice, as a battery that goes flat over a weekend.
There is a security dimension worth naming. A module in this family can request doors, ignition or engine start. A fault that leaves it publishing unreliable state is not something to leave indefinitely on a vehicle that is left unattended, and it is worth resolving properly rather than by disconnecting something.
Likely causes
Start here
- Module not coded to the vehicle · specialist
- Second hand module locked to another vehicle · specialist
- Corroded or loose connector · easy
- Poor wiring splice in an aftermarket installation · moderate
Less common
- Damaged data bus wiring · moderate
- Incorrect module configuration after replacement · specialist
- Another module holding the bus down · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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