When the power‑steering warning lamp flashes or stays on, the steering wheel may feel heavier than normal, especially at low speeds. Some drivers report a momentary “loss of assist” that returns after a few seconds of turning. In many cases the Service Engine Soon (or Check Engine) light also illuminates because the power‑steering module is part of the vehicle’s communication network. The problem does not affect engine output or brake performance, but it can compromise handling and driver confidence.
The power‑steering module communicates over the CAN bus. Moisture, road salt, or physical abrasion can corrode the connector pins or damage the harness. Corrosion changes the resistance of the signal lines, causing the control unit to receive data that falls outside the expected voltage range, which the network flags as “invalid data.”
Power‑steering modules contain a microcontroller, power‑stage drivers, and memory for calibration data. Overheating, exposure to water, or age‑related solder fatigue can cause the microcontroller to output corrupted frames. When the ECU reads these frames, it registers the U0420 fault.
Manufacturers release calibration updates to align steering‑assist torque with vehicle speed and load. If the module’s flash memory becomes corrupted—through a bad flash, an incomplete reflash, or a previous failed programming attempt—the module will send data that the network cannot interpret.
The steering module requires a stable 12 V supply. A weak battery, alternator voltage fluctuation, or a short to ground can produce momentary voltage spikes. The module may interpret these spikes as invalid data, prompting the fault code.
A loose ground strap or a cracked power feed can intermittently disconnect the module from the vehicle’s electrical system. The resulting loss of reference voltage makes the module’s output appear nonsensical to the network.
Connect a professional scan tool capable of reading manufacturer‑specific modules. Record the freeze‑frame data, then clear the code to see if it returns immediately.
Use the scan tool’s “Network Scan” function. Verify that Power‑Steering Module A is listed as “online.” If the module shows as “offline” or “invalid data,” proceed to wiring inspection.
Locate the power‑steering module—usually mounted near the steering rack or under the dash. Check the connector pins for corrosion, bent contacts, or broken wires. Clean any oxidation with a contact‑cleaner spray and reseat the connector.
Measure the module’s supply voltage with the ignition on. It should read between 12.2 V and 14.8 V. Check the ground resistance; it must be less than 0.1 Ω. Replace any damaged power or ground wires.
Some manufacturers provide a built‑in self‑diagnostic routine. Initiate it via the scan tool; a successful test indicates internal electronics are functional.
If the module passes the self‑test but the code persists, download the latest steering‑assist calibration from the manufacturer’s service portal and flash it to the module. Verify the flash completes without errors.
Remove the module and connect it to a bench‑test harness. A qualified technician can simulate CAN‑bus traffic to confirm whether the module still sends corrupted frames.
When the bench test fails, the internal circuitry is likely damaged. Install a new, VIN‑matched power‑steering module, program it to the vehicle’s specifications, and clear all codes. Expect labor of 1–2 hours; total shop cost typically ranges from $650 to $950, including programming.
Cost Overview
Repairing a failed microcontroller or re‑soldering board traces can be a temporary fix, especially when moisture ingress has already compromised the module’s interior. The cost of a professional repair often approaches $400, and the repaired unit may fail again within months. In contrast, a new, VIN‑matched module restores full functionality, includes the latest software, and carries a warranty that protects against future failures.
Modern control modules are complex and integrated with security and immobilizer systems. That’s why choosing a replacement isn’t only about the hardware—it’s about correct programming and compatibility. Flagship One specializes in VIN‑matched control modules, providing a plug‑and‑drive solution backed by a lifetime warranty. Replacement units vary depending on production date and software version, so the correct module is matched by VIN before programming. The pre‑programmed unit arrives ready for installation, eliminating dealer‑only reflash steps and reducing vehicle downtime.
When the power‑steering module shows repeated communication faults, the risk of losing steering assist while driving outweighs the modest savings of a repair. Opting for a fresh, correctly programmed module restores confidence and ensures the steering system operates within factory tolerances.
Flagship One provides VIN-programmed, OEM engine and powertrain control modules backed by lifetime warranty. Units arrive pre-programmed to your vehicle’s specifications for plug-and-drive installation.