U0421 means a module on the vehicle network received data from the suspension control module that it could not accept. The generic definition is "Invalid Data Received From Suspension Control Module A." Some manufacturers use a more specific name: in GM's service information it appears as "Invalid Data Received from Automatic Level Control Module." Either way, the suspension module is powered and communicating, but at least one message it sent failed the receiver's checks.
Severity: depends on the suffix and the symptoms. Can you keep driving? Usually. If the vehicle sits level, rides normally and shows no suspension warning, it is often a stored or history fault. If the vehicle sits low, leans, refuses to change height, or shows a suspension fault message, treat it as a real fault and have it checked before towing or driving off-road.
Why other modules care about suspension data
On vehicles with air suspension or automatic level control, the suspension module shares its ride height and status data with other systems. The brakes and stability control use it to adjust how they intervene. Adaptive headlamps use it to keep the beams level. Driver-assist cameras and radars use it to correct their aim for the body's pitch. When one of those receivers decides the suspension data is invalid, it records U0421 even if the suspension itself is working.
That is why U0421 can be stored in a module that has nothing obvious to do with suspension, such as the ABS module or the front camera.
What the suffix tells you
Scan tools usually display the code with a failure type byte after a dash. On Land Rover and Range Rover vehicles the two most searched are U0421-68 and U0421-94.
- -68, event information. The receiving module is recording that it saw invalid data as an event, without a persistent fault. Stored alone, with no symptoms, it is often a transient message problem.
- -94, unexpected operation. The data describes suspension behaviour that does not fit what the receiver expects, such as a height change it was not told about. Pair this with the suspension module's own codes before deciding where the fault is.
- -00, no sub type information. No further detail. Read the suspension module's own codes.
Land Rover LR4: a known false report
Land Rover issued a technical bulletin, SSM72146, for the 2015 model year L319 Discovery 4, sold in North America as the LR4. It lists several network codes appearing together, including U0415-68, U0421-68, U0416-68 and U0241-00. The cause Land Rover gives is unusual: the refrigerated cooler box in the centre console stays active after the ignition is switched off, which generates back EMF, an electrical disturbance, and makes modules raise codes because they fail to get information from other sensors.
The fix in the bulletin is software rather than hardware: reflash the ABS module, using diagnostic software at DVD 139 Patch 17 or later. If you have an LR4 or a Discovery 4 of that period showing this group of codes with no suspension symptoms, check whether this update has been applied before replacing anything.
GM: history codes you can ignore
GM published a preliminary information bulletin, PIT5762F, dated January 2023, listing history codes that set because of a software anomaly on 2020 to 2023 Cadillac CT4 and CT5, 2021 to 2023 Cadillac Escalade, 2020 to 2023 Chevrolet Corvette and 2021 to 2023 Chevrolet Suburban, Tahoe and GMC Yukon. U0421-00 stored in the front camera module is on that list. GM says these history codes do not turn on any warning lamp or message and should be ignored when stored as history. If the same code is current rather than history, GM says to diagnose it normally.
Diagnosing a real U0421
- Read codes from every module, including the suspension module itself. A suspension module that stores its own sensor, compressor or valve codes is telling you where the actual fault is. U0421 in other modules is then a consequence.
- Check battery voltage and charging. Air suspension compressors draw heavy current, and a weak battery or poor ground can make the suspension module send corrupted data during compressor operation.
- Check for software updates for the suspension module and for the module that stored U0421. Invalid-data codes are among the most common reasons for manufacturer software releases.
- Inspect ride height sensors and linkages. A bent linkage or damaged sensor produces height readings that other modules may reject as implausible. Compare the four height readings on a scan tool with the vehicle on level ground.
- Check the suspension module's power, ground and network connections, especially where the module sits in a wet area such as under a seat, in the boot floor or near a wheel arch.
- Replace the suspension module last, and expect it to need programming and a ride height calibration afterwards.
Related codes
U0421 often appears alongside U0416, invalid data from the vehicle dynamics module, and other invalid-data codes from the same data exchange. When several invalid-data codes point at different senders at once, suspect a shared cause such as low voltage, a network problem or software, rather than several failed modules. The overview of U codes and module communication explains how to read a group of them, and pending, stored and permanent codes explains the difference between current and history status that matters so much for the GM bulletin above.
Checking ride heights properly
Ride height checks only mean something on level ground with the vehicle at its normal height setting, the doors closed and no unusual load. Most air suspension systems let you read each corner's height on a scan tool. Compare the four readings with each other and with the specification in the service information. One corner reading very differently from the others, or a reading that jumps while the vehicle is standing still, points to that corner's sensor or linkage. All four readings being plausible makes a sensor fault unlikely and moves the search towards software, power supply or the network.
Frequently asked questions
Can I drive with U0421?
If the vehicle sits level and there is no suspension warning, generally yes. If it is sitting low, leaning or showing a suspension fault message, have it checked first, especially before towing or off-road driving.
Why does my Range Rover show U0421-68?
Often because another module flagged suspension data as invalid for a moment. On the 2015 LR4 and Discovery 4, Land Rover traced a group of codes including U0421-68 to the centre console cooler box and fixed it with an ABS reflash.
Is U0421 the air suspension compressor?
Not directly. It means the data from the suspension module was rejected. A compressor fault normally sets codes in the suspension module itself; read those to find out.
Should I clear U0421 and see if it comes back?
If it is a history code with no symptoms, clearing it and watching is reasonable, and on some GM models it is exactly what the manufacturer advises. If it returns as a current code, diagnose it.
Can a wheel alignment or lift set U0421?
It can. Lifting a vehicle with air suspension on a workshop lift, or leaving it jacked up with the ignition on, lets the suspension see heights it does not expect. Many vehicles have a jack or workshop mode that tells the suspension module to stop adjusting. If the code appeared after tyre, alignment or underbody work, ask whether that mode was used, clear the code and check whether it returns during normal driving.
Sources: Land Rover technical bulletin SSM72146 (February 2015); GM Preliminary Information PIT5762F (January 2023), both published by NHTSA; SAE J2012 code definitions and failure type bytes.