U0420 means a module received data from the power steering control module that it judged invalid: out of range, inconsistent with other data, or flagged by the sender as unreliable. It belongs to the U04xx block for invalid data, distinct from the U01xx and U02xx lost-communication codes. The module that stores it is the complainer, most often the electronic brake control module or a driver-assist module, and the steering module is the one it is complaining about.
Severity: low to moderate. Can you keep driving? Usually. Steering assist itself is rarely affected. What gets disabled is whatever depends on steering data: stability control, lane keeping, adaptive cruise, or an automatic braking system, with a warning light. If steering assist is reduced as well, treat it as a drive to the shop.
Three manufacturer cases
- GM, 2016 to 2018 Chevrolet Malibu and 2017 to 2018 Buick LaCrosse (bulletin 17-NA-190, March 2018): the brake module stores U0420 present or in history. GM's cause is a power moding issue where the entire vehicle's systems are not on-line, which typically happens during remote start. The correction line reads: no repairs should be made.
- Mazda, 2017 to 2018 Mazda3 built in Japan (bulletin 15-003/20, September 2020): the Smart Brake Support warning light with U0420:68 stored, addressed by the bulletin's procedure for that model range.
- Jaguar Land Rover service messages on heavier-than-expected steering list this code among those caused by low battery voltage or wiring harness faults: shorts, opens or high resistance.
The lesson: read which module stored it and what the vehicle was doing, before assuming the steering module is bad.
Causes, in the order they are found
A normal event on the specific vehicle, such as GM's remote start case, where the code is history only and nothing is wrong.
Low system voltage during cranking or from a weak battery, which makes modules send default or flagged data.
A wiring or connector fault on the steering module's power, ground or bus, with resistance that corrupts messages.
A software mismatch after the steering module or its partner was flashed, on vehicles where a driver-assist module validates the steering module's software version.
A steering angle sensor calibration not performed after alignment or steering work, so the steering module sends data the brake module rejects.
The steering module, last.
A diagnostic order
- Read which module stored U0420 and whether it is current or history, and note what the vehicle was doing (remote start, cranking, a jump start). Check for a bulletin on the exact vehicle.
- Test the battery and charging at rest and under load.
- Check for other U codes on the same modules; several together point at shared power or the bus.
- Inspect the steering module's power, ground and bus connectors for corrosion, water and terminal tension.
- Verify steering angle sensor calibration and perform it if steering or alignment work was done.
- Compare software levels if a module was recently replaced or flashed.
- Clear and verify through several key cycles and a drive; a history-only code that does not return needs no repair.
Frequently asked questions
Can I drive with a U0420 code?
Usually. Steering assist is rarely affected; stability or driver-assist functions that use steering data may be disabled with a warning light until the code is resolved. If steering feels heavy as well, drive to a shop.
Why does my Malibu or LaCrosse have U0420 after a remote start?
Because on 2016 to 2018 Malibu and 2017 to 2018 LaCrosse, GM's bulletin 17-NA-190 attributes U0420 in the brake module to a power moding condition during remote start, when not all systems are on-line. GM's instruction is that no repair should be made.
Is U0420 the power steering module?
Rarely. The code means another module rejected the steering module's data, and the common reasons are low voltage, a wiring or connector fault, a missing steering angle calibration or a software mismatch. The module is last on the list.
How much does it cost to fix U0420?
Often nothing, when the cause is a normal event or a weak battery. A calibration or a software update is an hour at a dealer. A connector repair is under an hour. A steering module is a major part and comes last.
Related codes
- U0131: lost communication with the power steering control module, the "not talking at all" version.
- U0401 / U0422: invalid data from the engine or body module, stored in the steering module in GM's 17-NA-390 case.
- C-series steering angle sensor codes in the brake module.
- See U codes and module communication for how to read a group of network codes.
The code number itself tells you the family and the domain; how to read an OBD2 code number explains what each character means.