U0121 on 2017-2018 Chevrolet Bolt EV: A Harmless Software Glitch Explained

What's Unique About the 2017-2018 Chevrolet BOLT EV

Unlike most vehicles where a U0121 code points to a potentially serious wiring or module failure that disables the ABS and stability control, on the 2017-2018 Bolt EV it's a well-documented software bug. The issue originates in the Front View Camera Module (FVCM), which incorrectly reports a communication loss. This means that despite the alarming definition of the code, the vehicle's braking systems are typically unaffected. GM's official advice in TSB 16-NA-177 for the 2017 model year was for technicians to simply ignore the code, as it causes no customer concern and would be eliminated in the 2018 model year software.

Diagnostic Flowchart

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

Symptoms You May Notice

• No noticeable symptoms
• Code may be found only when scanning with an OBD-II tool
• No ABS, traction control, or other warning lights on the dashboard

Most Likely Causes

1. Front View Camera Module (FVCM) Software Anomaly 🔴 High Probability This is a documented issue specific to the 2017-2018 Bolt EV, as confirmed by GM Technical Service Bulletins PIT5599 and 16-NA-177. The FVCM software falsely logs the communication loss with each ignition cycle.
   How to confirm: The code will be present, often in history, without any corresponding ABS warning lights or drivability issues. A dealer can confirm if TSBs PIT5599 or 16-NA-177 apply to the vehicle's VIN. TSB 16-NA-177 specifically instructs technicians to ignore this history code.
   Typical fix: A dealership needs to reprogram the Front View Camera Module (FVCM) with updated software. Alternatively, following the advice in TSB 16-NA-177, the code can be ignored if no symptoms are present.
   Est. part cost: $0
2. Weak or Failing 12V Battery ⚪ Low Probability → Shop Vehicle Battery Like many modern EVs, the Bolt EV is sensitive to low voltage from the 12V accessory battery. A weak 12V battery can cause numerous, seemingly random communication codes between modules as they fail to initialize correctly.
   How to confirm: Test the 12V battery with a multimeter or battery tester. A healthy battery should read approximately 12.6 volts with the vehicle off. Readings below 12.2V suggest it is weak. Owners report symptoms like the car failing to power on, 'No Remote Detected' messages, and other electrical gremlins as precursors to 12V battery failure.
   Typical fix: Replace the 12V accessory battery. 🎬 Watch: Step-by-step guide to replacing your Bolt's 12V battery. The OEM battery is an AGM type.
   Est. part cost: $150-$250

Rare But Worth Checking

• True ABS Module or Wiring Failure: → Shop ABS Control Module While the software glitch is most common, a genuine hardware failure is still possible. This would typically be accompanied by an illuminated ABS warning light, stability control messages, and potentially other 'U' codes. Diagnosis would involve checking fuses, wiring, and connectors for the ABS module, including power and ground connections.

Diagnosis Steps

1. Check for Symptoms: Confirm that there are no ABS, traction control, or check engine warning lights on the dashboard. If there are, a more serious hardware issue may be present.
2. Scan for Codes: Use an OBD-II scanner to read the codes. Note if U0121 is the only code or if other communication codes are present. If U0121 is a history code and there are no symptoms, it is likely the known software bug.
3. Verify TSB Applicability: The primary cause is a known software bug. A dealer can check if TSBs PIT5599 or 16-NA-177 apply to your vehicle. TSB 16-NA-177 explicitly states to ignore the history code.
4. Check 12V Battery Health: If multiple 'U' codes are present or the vehicle exhibits other electrical oddities (e.g., slow startup, 'No Remote Detected' messages), test the 12V accessory battery. Ensure it has a static voltage of at least 12.4V-12.6V.
5. Seek Software Update: If the code is a nuisance and you want it gone, schedule an appointment with a Chevrolet dealer to have the FVCM software updated per the relevant TSB. This is often not covered by warranty unless the vehicle is still within its bumper-to-bumper period.
6. Inspect ABS System (if symptoms are present): Only if ABS warning lights are on, proceed with standard U0121 diagnostics: 🎬 See this video for a deep dive into U0121 diagnostics. check ABS fuses, inspect the wiring and connector at the ABS module for damage or corrosion.

Parts You'll Likely Need

• No Parts Required - Software Update — The most common cause is a software anomaly that is resolved by reprogramming a module, not replacing a part. In many cases, the official guidance is to ignore the code entirely.
  OEM price range: $0
  Aftermarket price range: $0
• 12V AGM Accessory Battery (OEM #ACDelco LN1AGM (GM Part #88866268)) — In the less common case that a weak 12V battery is causing communication faults, it will need replacement. The Bolt EV uses a BCI Group Size LN1 AGM battery.
  Trusted brands: ACDelco, DieHard
  OEM price range: $200-$315
  Aftermarket price range: $150-$250

Related Codes That Often Appear With This One

• U0140 — In cases of a failing 12V battery, multiple communication loss codes, such as U0140 (Lost Communication with Body Control Module), can appear alongside U0121.
• P1E00 — This code, often related to the hybrid/EV powertrain control module, can also appear in conjunction with other communication faults when the 12V battery is failing.

Technical Service Bulletins (TSBs) & Recalls

• PIT5599: Communicates a false setting of DTC U0121 in the FVCM due to a software anomaly. States the code will set and be moved to history with each ignition cycle.
• 16-NA-177: Explains DTC U0121 sets due to a software anomaly and does not turn the MIL on or change vehicle performance. Explicitly advises technicians to 'Ignore this code as it causes no customer concern or vehicle issue' and notes it will be eliminated in the 2018 model year.

Platform-Specific Known Issues

• A software anomaly in the Front View Camera Module (FVCM) can cause a false U0121 code to be set in history with each ignition cycle, as documented in GM TSB #PIT5599.
• GM TSB #16-NA-177 clarifies that this specific software-induced U0121 code does not turn on the Malfunction Indicator Lamp (MIL) or affect vehicle performance, and advises technicians to ignore the history code for the 2017 model year.

Mechanic-Grade Diagnostic Values

• High-Speed CAN Bus Resistance — expected: Approximately 60 Ω (Ohms). Failure: A reading of ~120 Ω indicates an open circuit or missing terminating resistor. A reading significantly below 60 Ω suggests a shorted wire or an extra terminating resistor on the bus.
• CAN Bus Voltage (Key On, Engine Off) — expected: CAN High (Pin 6 at OBD-II) fluctuates 2.5V-3.5V. CAN Low (Pin 14 at OBD-II) fluctuates 1.5V-2.5V. A multimeter may show an average around 2.7V for High and 2.2V for Low.. Failure: Voltages stuck high, low, or at 0V indicate a wiring fault (short to power/ground or open).

Scan Tool Commands That Help

• GM GDS2 / SPS2 (Service Programming System): Module Reprogramming: K173 Front View Camera Module — This is the official fix for the software anomaly described in TSBs PIT5599 and 16-NA-177. It updates the FVCM firmware to prevent the false U0121 code from being logged. This can be performed by a dealer or by a DIYer with a J2534 tool (like a VXDIAG VCX Nano) and a per-VIN subscription to GM's programming software via ACDelco TDS.

Wiring & Ground Locations

• K17 Electronic Brake Control Module (EBCM) — Under the hood, located on the passenger side, below the cabin heater assembly. 🎬 Watch: How to locate the ABS and traction control modules. It is identifiable as a black control box attached to a metal block with six brake lines connected to it.. In the rare event of a true hardware failure (indicated by ABS warning lights), this is the module that has lost communication. Its connector and nearby ground points would be the primary focus for inspection for corrosion or damage.
• OBD-II Data Link Connector (DLC) — Inside the cabin, typically under the driver's side dashboard.. This is the primary access point for network diagnostics. The High-Speed GMLAN (CAN bus) is accessible at Pin 6 (CAN High) and Pin 14 (CAN Low) for resistance and voltage measurements to test the health of the entire network.

Real Owner Repair Stories

• VXdiagshop.com Blog / User Review (2017 Chevrolet Bolt EV) — User was replacing and reprogramming the radio module, not the FVCM, but the procedure is identical.
  ❌ Tried (didn't work) A simple part swap was insufficient; the new module had incorrect features (like heated seats for a car without them) and CarPlay would not activate, requiring programming.
  ✅ What actually fixed it The user purchased a VXDIAG VCX Nano GM scanner ($109) and a two-year GM SPS2 programming subscription for their VIN ($45). Using this equipment, they successfully connected to the car and applied the correct software update to the module, restoring full functionality. This demonstrates that the official software fix for the U0121 FVCM issue is accessible to advanced DIYers.

OEM Part Supersession History

• 42489890 → — This is the listed OEM part number for the Front View Camera Module for the 2017-2018 Chevrolet Bolt EV.
  Heads up: This part requires programming and setup using GM's Service Programming System (SPS2) after installation to ensure proper operation and to load the corrected software that prevents the false U0121 code.

Model Year Variations Within This Range

• 2017 vs. 2018: TSB #16-NA-177, dated early 2017, states the software anomaly 'will be eliminated in the 2018 model year.' This implies that while early production 2018 models may still have the faulty software, later 2018 models should have the fix from the factory. Therefore, encountering a U0121 on a late-2018 Bolt EV (without other codes/symptoms) is less likely to be the known software bug and may warrant a closer look at other potential causes.