U0420 on 2018-2020 Mazda CX-9: Power Steering & Sensor Fault Causes and Fixes

What's Unique About the 2018-2020 Mazda CX-9

For this specific generation of Mazda CX-9 and its platform mates, the U0420:68 code is strongly linked to a known software sensitivity. A temporary voltage drop during engine cranking, often from a slightly weak or aging battery, can cause the Electric Power Steering (EPS) module to fail its self-check. The Forward Sensing Camera, which relies on steering angle data for safety features, detects this communication error and sets the code, triggering a cascade of warning lights for seemingly unrelated systems like Smart Brake Support.

Diagnostic Flowchart

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

Symptoms You May Notice

• Multiple warning lights on the dashboard, including the amber SBS/SCBS light, power steering warning, and TCS/DSC light.
• Dashboard messages such as 'Front Camera Sensor System Malfunction' or 'Power Steering Malfunction'.
• Additional dashboard messages may include 'Forward Smart City Brake Support Malfunction' and 'Driver Attention Alert System Malfunction 🎬 Watch: See what these dashboard warning messages look like in action.'.
• Complete or intermittent loss of power steering assist, making the steering wheel feel very heavy.
🎬 Try this quick reset method for your power steering light.
• Vibration, shaking, or a clicking sound felt through the steering wheel when the fault is active.
• Disabling of i-ACTIVSENSE safety features like Mazda Radar Cruise Control (MRCC) and Lane-keep Assist System (LAS).

Most Likely Causes

1. EPS/FSC Software Glitch 🔴 High Probability A specific Technical Service Bulletin (15-003/20) was issued for this exact problem, where low battery voltage on startup causes the EPS module to fail its initialization, which is then flagged by the Forward Sensing Camera (FSC) as DTC U0420:68.
   How to confirm: A Mazda dealer can check if TSB 15-003/20 applies to the vehicle's VIN and if the software update has been performed. The fault almost always occurs immediately after starting the engine.
   Typical fix: Reprogramming the Forward Sensing Camera (FSC) module with updated software using the Mazda Modular Diagnostic System (M-MDS). The labor time for this warranty repair is listed as 0.3 hours.
   Est. part cost: $0
2. Weak or Failing Battery 🟡 Medium Probability → Shop Vehicle Battery This is the direct trigger for the software glitch. Modern vehicles with high electrical demands are highly sensitive to stable voltage, and the EPS system's self-test during cranking is a key point of failure if voltage drops excessively. Many owners in forums report a new battery resolving a host of electrical gremlins.
   How to confirm: Perform a professional load test on the battery. A simple voltage check is insufficient; a healthy battery should show at least 12.6 volts when fully charged and hold its voltage under load. Many auto parts stores offer this service for free.
   Typical fix: Replace the vehicle's 12V battery. Consider an AGM (Absorbed Glass Mat) battery 🎬 See this step-by-step guide to replacing your CX-9 battery. for better performance and longevity in vehicles with high electrical loads.
   Est. part cost: $150-$400
3. Damaged Wiring or Corroded Connector ⚪ Low Probability The EPS module is located on the steering rack, exposing its connectors to potential moisture, road salt, and debris which can lead to corrosion.
   How to confirm: Visually inspect the wiring harness and electrical connector at the Electric Power Steering module on the steering rack. Look for green/white corrosion, bent pins, or physical damage to the wires.
   Typical fix: Clean the connector with electrical contact cleaner or repair the damaged section of the wiring harness. Ensure the connection is secure.
   Est. part cost: $10-$100

Rare But Worth Checking

• Failed Electric Power Steering (EPS) Control Module: → Shop Power Steering Control Module While less common than the software issue, an internal failure of the EPS module itself can cause it to send invalid data. This is typically diagnosed after all other possibilities (software, battery, wiring) have been ruled out.
• Failed Forward Sensing Camera (FSC): → Shop Collision Avoidance Sensor In rare cases, the FSC module itself may fail. Replacement is expensive, with dealer quotes often exceeding $1,200. It's critical to note that used or salvage FSCs are typically VIN-locked and will not function, requiring the purchase of a new part and dealer programming/calibration.

Diagnosis Steps

1. Check for applicable Technical Service Bulletins, specifically Mazda TSB #15-003/20, to see if the vehicle's VIN is within the affected range.
2. Perform a comprehensive battery and charging system test, including a load test, not just a voltage check.
3. Using a professional scan tool, check for DTCs in all relevant modules, paying close attention to the specific suffix code (e.g., U0420:68).
4. If TSB 15-003/20 is applicable and has not been performed, have a Mazda dealer reprogram the FSC module. The TSB advises using a battery charger during reprogramming to ensure stable voltage.
5. If the code persists, raise the vehicle safely and visually inspect the electrical connector and wiring harness at the EPS module (located on the steering rack) for corrosion, moisture, or damage.
6. Verify the power and ground connections to the EPS module are clean, tight, and have correct voltage/resistance. A good ground should measure less than 0.1 Ω.
7. If all other steps fail, and especially if a code like U3000 is also present, diagnosis of the FSC or EPS module for internal failure may be necessary.

Parts You'll Likely Need

• Vehicle Battery (OEM #BCI Group Size 35) — A weak battery is the primary trigger for the software fault that causes this code on startup. An AGM (Absorbed Glass Mat) battery is recommended for its ability to handle high electrical loads and prevent the voltage drop that causes the issue.
  Trusted brands: Panasonic (OEM), Duralast Platinum AGM, Optima Yellow Top (AGM), Interstate
  OEM price range: $250-$400
  Aftermarket price range: $150-$350

Related Codes That Often Appear With This One

• U0131 — This code means 'Lost Communication With Power Steering Control Module'. It often appears alongside U0420, indicating a more severe communication breakdown rather than just invalid data.
• C0051 — This code relates to the Steering Wheel Angle Sensor. The EPS module uses this data, and if the data is corrupt (triggering U0420), the ABS/DSC module may also flag a fault with the steering angle signal.
• U3000:04 — This code indicates an internal malfunction of the FSC itself. If this code is present along with U0420, it points more strongly toward a hardware failure of the camera module rather than just a software glitch.

Technical Service Bulletins (TSBs) & Recalls

• 15-003/20: SBS/SCBS WARNING LIGHT ON WITH DTC U0420:68.

Platform-Specific Known Issues

• Mazda Technical Service Bulletin 15-003/20 directly addresses DTC U0420:68 on 2018-2020 CX-9 models, as well as some CX-5, Mazda6, and Mazda3 vehicles. It identifies a software issue where the Forward Sensing Camera (FSC) logs this fault due to an EPS initialization error during engine start, caused by a momentary voltage drop. The prescribed fix is to reprogram the FSC module.

Mechanic-Grade Diagnostic Values

• EPS Module Supply Voltage — expected: 12.2 V to 14.8 V with ignition on. Failure: Voltage below 12.2V, especially during cranking, can trigger the fault.
• EPS Module Ground Resistance — expected: Less than 0.1 Ω. Failure: Higher resistance indicates a poor ground connection, which can cause intermittent communication issues.
• Battery Voltage (Static) — expected: At least 12.6 V when fully charged. Failure: A reading below 12.4V suggests a discharged or failing battery that may not provide sufficient cranking voltage.

Hidden / Shadow Codes Worth Checking

• U0420:68: This specific sub-code, logged by the Forward Sensing Camera (FSC), Adaptive Front Lighting System (AFS), and other modules, means 'Error signal received from electric power steering (EPS) control module'. It is an 'Event Information' code, indicating the module is reporting a fault it has received from another module, rather than an internal fault. (see via A professional scan tool like the Mazda M-MDS is required to read suffix codes. Basic OBD-II readers will typically only show 'U0420'.)

Scan Tool Commands That Help

• Mazda Modular Diagnostic System (M-MDS): Module Reprogramming > FSC — This is the specific function required to perform TSB 15-003/20. It updates the Forward Sensing Camera's software to be less sensitive to the EPS initialization error on startup.
• Mazda Modular Diagnostic System (M-MDS): Work Support > i-ACTIVSENSE > FSC Aiming — This calibration procedure is mandatory after replacing the Forward Sensing Camera (FSC) or the windshield. It can be done dynamically (driving) or statically (in the shop with targets). Failure to perform aiming will result in DTCs like B14F0:76 and non-functional safety systems.

Wiring & Ground Locations

• Electric Power Steering (EPS) Module — Mounted directly on the steering rack and pinion assembly, located low in the front of the vehicle on the subframe.. This location exposes the module and its electrical connector to road salt, moisture, and debris, making it a primary point to inspect for corrosion or wiring damage that could cause the 'Invalid Data' signal.
• Forward Sensing Camera (FSC) Module — Located at the top center of the windshield, integrated into the rearview mirror mounting assembly.. This is the module that most commonly logs the U0420:68 code. Its operation is critical, and any work on the windshield requires its recalibration.
• Ground Points — Wiring diagrams for the 2019 CX-9 show multiple ground points in the engine room and chassis. The specific ground for the EPS is part of its main harness connected to the subframe/chassis.. A loose or corroded ground for the EPS module can cause fluctuating reference voltage, leading to invalid data signals being sent over the CAN bus.

Real Owner Repair Stories

• Reddit user on r/MazdaCX9 (2021 Mazda CX-9 Carbon Edition) — Multiple warning lights for front camera system, Smart Brake Support, and others.
  ❌ Tried (didn't work) Initial diagnostics did not immediately pinpoint the cause.
  ✅ What actually fixed it The dealer diagnosed a failed Forward Sensing Camera (FSC) and replaced it. The part number was GSH7-67-XCXV. The repair, including the part ($971) and labor for installation and calibration, was quoted at approximately $1,200 and was covered under warranty.

OEM Part Supersession History

• TN39-67-RC0 → TN39-67-RC0A — Part revision or update by the manufacturer.
  Heads up: This is a camera for the 360-view monitor system and is different from the primary Forward Sensing Camera (FSC) for i-ACTIVSENSE. It's critical to order parts by VIN.
• KEME-67-XCX → KEME-67-XCXA — Part revision or update by the manufacturer.
  Heads up: This Forward Sensing Camera is listed for the CX-90 model, highlighting that different platforms use different camera part numbers. Using an incorrect part number will not work.
• TK48-32-110 → Unknown — This is the part number for the entire steering rack assembly for 2016-2023 models. While not a direct cause of the software glitch, it would be replaced if the EPS module itself failed.
  Heads up: Replacement of the steering rack is a major repair and should only be done after confirming an internal, unfixable hardware failure of the integrated EPS module.

Model Year Variations Within This Range

• 2018-2020: TSB 15-003/20 specifies VIN cutoffs for affected vehicles. For the CX-9, it applies to vehicles with VINs lower than JM3TC**423432, produced before June 23, 2020. Vehicles produced after this date should have the updated software from the factory.