Invalid Data from EPS Module
The U0420 code indicates that the vehicle's main control module (typically the PCM or CAN network master) has received invalid, corrupt, or out-of-range data from the Electric Power Steering Control Module (EPS/PSCM) over the serial communication bus. This diagnostic trouble code sets when the receiving module checks message integrity via redundancy or checksum and detects inconsistencies that prevent proper data processing. Per SAE J2012, this is a network communication code related to invalid data received from a specific module.
1. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.
2. Most Common Causes
- Internal failure of the Power Steering Control Module (PSCM/EPS)
- Electromagnetic interference or noise on the CAN bus
- Abnormal resistance in CAN bus terminators
- Corrosion, oxidation, or damage in PSCM harness connectors
- Voltage variation in the PSCM due to weak battery or faulty alternator
- Outdated or corrupted PSCM software
- Fault in the master control module that interprets the messages
3. Symptoms You Will Notice
- Power steering warning light illuminated
- Loss of power steering assist or intermittent assist
- Increased steering effort, especially at low speeds
- Possible limp mode or reduced vehicle performance
4. Step-by-Step Diagnosis
Scan for all stored codes and freeze frame data to confirm U0420 and related codes.
Inspect the CAN bus wiring and connectors at the PSCM for damage, corrosion, or loose pins.
Check battery voltage and charging system output; ensure stable voltage to the PSCM.
Test CAN bus resistance and signal integrity with an oscilloscope or multimeter.
If wiring and voltage are OK, attempt to reprogram or update the PSCM software.
If the issue persists, replace the PSCM and perform necessary programming.
5. Frequently Asked Questions
Is it safe to drive?▼
No, it is not safe to drive with code U0420. The power steering may fail unexpectedly, increasing the risk of an accident, especially at low speeds or during parking maneuvers.
How much does it cost to fix?▼
Repair costs typically range from $200 to $1,200 USD, depending on the root cause. Simple wiring repairs may cost under $300, while PSCM replacement and programming can exceed $1,000.