U0420 on 2017-2020 Mazda CX-5: Invalid Data from Power Steering Fixes

What's Unique About the 2017-2020 Mazda CX-5

On this generation of Mazda CX-5, the U0420 code is strongly linked to a known software sensitivity in the advanced driver-assistance systems (ADAS). A momentary voltage drop during engine start-up, often from a slightly weak or aging battery, can cause the Electric Power Steering (EPS) module to fail its initial self-check for the steering center position. The Forward Sensing Camera (FSC), which relies on steering data for safety features like Smart Brake Support (SBS), detects this brief communication error and sets the code, triggering a cascade of warning lights for seemingly unrelated systems.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Amber SBS/SCBS (Smart Brake Support/Smart City Brake Support) warning light is on.
• Warning messages in the instrument cluster, such as 'Front camera sensor system malfunction' or 'Forward Smart City Brake Support Malfunction'.
• A 'Driver attention alert system malfunction' message may also appear.
• TCS/DSC (Traction/Dynamic Stability Control) indicator light is on.
• Power steering warning light may be illuminated.

Most Likely Causes

1. Forward Sensing Camera (FSC) Software Anomaly 🔴 High Probability → Shop Collision Avoidance Sensor A specific TSB (#15-003-20-3737) identifies a software issue where the FSC is too sensitive to a temporary EPS initialization error during engine cranking. This affects not only the CX-5 but also the Mazda3, Mazda6, and CX-9 of the same era.
   How to confirm: A Mazda dealer can check if the vehicle's VIN is covered by the TSB and if the software update has been performed. This is the first step in diagnosis. The TSB applies to CX-5 models with VINs lower than JM3KF**796943 (produced before February 4, 2020).
   Typical fix: Reprogram the Forward Sensing Camera (FSC) module with updated software using the Mazda Modular Diagnostic System (M-MDS). The TSB specifies using IDS 108.04 or a later software version.
   Est. part cost: $0
2. Weak or Failing Battery 🟡 Medium Probability → Shop Vehicle Battery The software glitch is often triggered by a momentary voltage drop during engine start, which is a classic symptom of an aging battery. Modern vehicles like the CX-5 have high electrical demands for their various modules, making them very sensitive to battery condition.
   How to confirm: Perform a comprehensive battery and charging system test using a digital load tester, not just a simple voltage check. Voltage should remain stable and above ~10V during cranking. Many auto parts stores offer this service for free.
   Typical fix: Replace the vehicle battery. 🎬 Watch: A simple walkthrough for replacing your CX-5 battery. An AGM (Absorbed Glass Mat) battery of the correct group size (BCI Group Size 35) is often recommended to better handle the high electrical demands and prevent the voltage drop that triggers the fault.
   Est. part cost: $150-$350
3. Electric Power Steering (EPS) Module Malfunction ⚪ Low Probability → Shop Power Steering Control Module
   How to confirm: If the FSC software is up to date and the battery is healthy, the EPS module itself must be diagnosed. This involves checking for other EPS-specific codes (like C-codes) and analyzing its live data with a professional scan tool.
   Typical fix: Replace the Electric Power Steering module, which is often integrated with the steering rack. This is an expensive repair and should only be done after ruling out software and battery issues.
   Est. part cost: $800-$1500

Rare But Worth Checking

• Wiring or Connector Issue: Corrosion or damage to the CAN bus wiring between the FSC and the EPS module can corrupt the signal. This is worth checking if the vehicle has a history of water damage or physical impact. A visual inspection of connectors for corrosion or bent pins is a good starting point.
• Forward Sensing Camera (FSC) Hardware Failure: → Shop Collision Avoidance Sensor If a code like U3000:04 (Internal Malfunction) is present alongside U0420, it points towards a hardware failure of the camera module itself, rather than just a software glitch. One owner on a forum reported a dealer quote of ~$1750 CAD to replace the FSC for this code.
• Forward Sensing Camera (FSC) Overheating: → Shop Collision Avoidance Sensor While not directly causing U0420, it's a known issue that highlights the sensitivity of the FSC module. TSB 15-003/20 notes that FSC reprogramming may fail if the camera is too hot, and its temperature can be monitored via the M-MDS scan tool. Another TSB for other models (15-002/18) addresses cameras shutting down in hot weather, requiring an updated part. This indicates the camera itself can be a point of failure.

Diagnosis Steps

1. Verify the customer complaint and check for warning lights on the dash.
2. Scan all vehicle modules for DTCs using a professional scan tool capable of reading Mazda-specific codes and suffixes (e.g., U0420:68). Note which module(s) have stored the code.
3. Check for applicable Technical Service Bulletins, specifically Mazda TSB #15-003-20-3737, to see if the vehicle's VIN is within the affected range (lower than JM3KF**796943).
4. Perform a comprehensive battery and charging system test, including a load test, to rule out low voltage during cranking as the trigger.
5. If TSB #15-003-20-3737 applies and the update has not been performed, have a Mazda dealer reprogram the FSC module. The TSB explicitly warns to connect a battery charger to the vehicle to maintain stable voltage during the update procedure.
6. After the update, clear all DTCs and perform a test drive to verify the fix. This may include a steering angle sensor reset procedure (turning the wheel lock-to-lock 🎬 See how to perform a steering position sensor reset.).
7. If the code returns, further diagnosis of the CAN bus network, wiring, and connectors between the FSC and EPS module is required.
8. If the network is intact, suspect a hardware failure of the EPS module or, if DTC U3000 is also present, the FSC module.

Parts You'll Likely Need

• Vehicle Battery (OEM #BCI Group Size 35) — A weak battery is the most common trigger for the software fault that causes this code on startup. Replacing it often prevents the issue from recurring, even before a software update.
  Trusted brands: Panasonic (OEM), Duralast Platinum AGM, Optima Yellow Top (AGM), Interstate
  OEM price range: $250-$400
  Aftermarket price range: $150-$350
• Forward Sensing Camera (FSC) (OEM #KDD1-67XCX (older, may fail), B61L-67XCX-T (newer replacement)) — Only needed if diagnosis confirms a hardware failure of the camera itself, often indicated by code U3000:04. A software update will not fix a hardware fault. Replacement requires mandatory professional calibration. 🎬 Watch: How a professional ADAS calibration is performed.
  Trusted brands: Mazda (OEM)
  OEM price range: $1200-$1700
  Aftermarket price range: N/A

Related Codes That Often Appear With This One

• U0131 — This code means 'Lost Communication With Power Steering Control Module' and indicates a more severe communication failure than just invalid data.
• C0051 — This code relates to the Steering Wheel Angle Sensor. If the EPS module sends corrupt data (triggering U0420), the ABS/DSC module may also flag a fault with the steering angle signal it's receiving.
• U3000:04 — This code indicates an 'Internal Malfunction' of a control unit. If logged by the FSC alongside U0420, it strongly suggests a hardware failure of the camera itself, making the software update irrelevant.

Technical Service Bulletins (TSBs) & Recalls

• 15-003-20-3737: SBS/SCBS WARNING LIGHT ON WITH DTC U0420:68. This TSB is the primary source identifying the software issue between the FSC and EPS modules. It also applies to 2017-18 Mazda3, 2017-20 Mazda6, and 2018-20 CX-9.

Platform-Specific Known Issues

• Mazda Technical Service Bulletin #15-003-20-3737 directly addresses DTC U0420:68 on 2017-2020 CX-5 models. It confirms that a momentary voltage drop during cranking can cause the EPS module to fail its 'initial learning for the steering center position'. This error is then detected and flagged by the Forward Sensing Camera (FSC), leading to the code and warning lights. The official fix is a software update for the FSC.

Mechanic-Grade Diagnostic Values

• CAN Bus Line Voltage (Idle) — expected: CAN High (CAN_H) and CAN Low (CAN_L) lines should both measure approximately 2.5V at the module connector with key on, engine off.. Failure: Significant deviation from 2.5V, or one line being shorted to ground (0V) or power (5V/12V), indicates a network wiring fault.
• CAN Bus Termination Resistance — expected: Approximately 60 ohms when measured across CAN_H and CAN_L pins with the battery disconnected.. Failure: A reading of ~120 ohms indicates one of the two terminating resistors is missing or the circuit is open. A reading near 0 ohms indicates a short between the two lines.
• EPS Module Supply Voltage — expected: 12.2V to 14.8V with the ignition on.. Failure: Voltage below 12.2V, especially a significant drop during engine cranking, can trigger the initialization fault described in the TSB.
• EPS Module Ground Resistance — expected: Less than 0.1 Ω between the module's ground pin and a clean chassis ground.. Failure: Higher resistance points to a corroded or loose ground connection, which can cause intermittent communication errors.

Hidden / Shadow Codes Worth Checking

• U0420:68: This specific suffix code means 'Error signal received from electric power steering (EPS) control module'. It is logged by the Forward Sensing Camera (FSC) and is the primary code identified in TSB 15-003-20-3737. It's an 'Event Information' code, meaning the FSC is reporting a problem it heard from the EPS, not an internal FSC fault. (see via A professional scan tool like the Mazda M-MDS or a high-end aftermarket scanner (like some FORScan setups) is required to read suffix codes. Basic OBD-II readers will only show 'U0420'.)

Scan Tool Commands That Help

• Mazda M-MDS: Module Reprogramming > FSC — This is the specific function required to perform the software update outlined in TSB 15-003-20-3737. It flashes the Forward Sensing Camera with new software to make it less sensitive to the startup voltage drop.
• Mazda M-MDS / FORScan / other professional ADAS tools: FSC Aiming / Calibration — This calibration is mandatory after replacing the Forward Sensing Camera or the windshield. It can be a static procedure using targets or a dynamic one done by driving the vehicle. Failure to calibrate will result in ADAS system malfunctions.
• Manual Procedure / Scan Tool: Steering Angle Sensor (SAS) Initialization/Reset — This procedure is required anytime the battery is disconnected or goes completely flat. It recalibrates the steering wheel's center position for the DSC/EPS systems. The manual method involves turning the ignition on (engine off) and turning the steering wheel fully from lock to lock.

Wiring & Ground Locations

• EPS Control Module (CM) — The EPS Control Module is integrated into the steering wheel and column assembly, located under the dashboard.. This module is the source of the 'invalid data'. While usually not faulty itself, its power and ground connections are critical. The main power is from the 'EPS 60A' fuse and it has a dedicated ground point.
• Forward Sensing Camera (FSC) — Located at the top center of the windshield, integrated into the rearview mirror mount.. This is the module that most often logs the U0420:68 code. It receives data from the EPS module and will disable safety systems like SBS/SCBS if the data is invalid. Its physical connection to the windshield mount is critical.
• DLC-2 (Data Link Connector) — The standard 16-pin OBD-II port, located under the driver's side dashboard.. This is the primary access point for a scan tool to communicate with all modules. Technicians can test CAN bus voltages (Pins 6 & 14) and network resistance here to get a general idea of the communication bus health.

Real Owner Repair Stories

• Mazda Forum User (2017 Mazda CX-5 (shares platform components)) — Multiple warning lights on, blind spot and lane change systems would not calibrate.
  ❌ Tried (didn't work) The windshield replacement company (Safelite) checked fuses and connections without success.
  ✅ What actually fixed it The Mazda dealer discovered the Forward Sensing Camera (FSC) was not seated properly on its bracket on the new windshield. Correctly mounting the camera and then performing the calibration procedure resolved all the warning lights and communication issues.

OEM Part Supersession History

• KDD1-67-XCX → B61L-67-XCX-T (and other B61L-67-XCX revisions) — The original KDD1 series cameras may be more prone to internal failure (leading to U3000:04) or the sensitivity issue addressed by TSBs. The B61L series is the updated replacement part.
  Heads up: While a used B61L camera may physically fit in place of a KDD1, it is critical to ensure it is compatible with the vehicle's options. After installation, a mandatory FSC aiming/calibration procedure must be performed with a compatible scan tool.

Model Year Variations Within This Range

• 2017-2020 (pre-refresh): The TSB #15-003-20-3737 applies specifically to 2017-2020 CX-5 models with VINs lower than JM3KF**796943, which were produced before February 4, 2020. Vehicles produced after this date should have the updated FSC software from the factory and are less likely to experience this specific issue.