// blog # Read & Reset OBD-II, CAN and CAN-FD Codes (Step-by-Step, 2025) Identify, interpret and clear DTCs. Examples of common P-codes, live data logging and safe reset workflow for modern vehicles. Updated 14 Aug, 2025 [← All posts](https://www.autopi.io/blog/) The Check Engine Light means that the vehicle onboard diagnostics system has detected a fault or an abnormal operating condition. The fault can be simple, like a loose fuel cap. But it can also point to a problem that need proper diagnosis. This guide explains what the Check Engine Light means, how diagnostic trouble codes are read, when codes can be cleared, and how AutoPi hardware and AutoPi Cloud can be used to monitor fault codes across a vehicle or fleet. The important point is simple: clearing a fault code does not repair the vehicle. The code should be read, understood and checked against live data before it is cleared. [ Learn more ](https://www.autopi.io/) ## What is the Check Engine Light? The Check Engine Light is a warning from the vehicle diagnostic system. It is normally shown as an engine-shaped icon or as text on the dashboard. When the engine control unit detects a fault, it stores a diagnostic trouble code. This is also called a DTC. The code gives a starting point for troubleshooting. It does not always tell which part has failed directly. Common causes include: - **Loose or damaged fuel cap:** Can trigger an EVAP leak code on some vehicles. - **Misfire:** Can be caused by spark plugs, ignition coils, injectors, compression issues or fuel delivery problems. - **Oxygen sensor fault:** Can affect fuel trim and emissions control. - **Mass airflow sensor issue:** Can affect air-fuel calculation and how the vehicle drives. - **Catalytic converter efficiency fault:** Can indicate converter wear, sensor issues, exhaust leaks or upstream combustion problems. A steady Check Engine Light usually means that the vehicle can still be driven carefully. But the code should be read as soon as possible. A flashing Check Engine Light is more serious. It often means a misfire that can damage the catalytic converter. In this case, reduce load and avoid continued driving until the problem is checked. An [AutoPi CAN FD Pro](https://www.autopi.io/hardware/autopi-canfd-pro/) or another compatible diagnostic tool can be used to read fault codes and collect extra data around the fault. [ View AutoPi CAN FD Pro ](https://www.autopi.io/hardware/autopi-canfd-pro/) ## Common Diagnostic Trouble Codes (DTCs) The table below shows common OBD-II codes and typical areas to check first. These are not guaranteed root causes. A proper diagnosis should also include live data, freeze-frame data, wiring checks and vehicle-specific service information where needed. | DTC code | Description | Common areas to check | First diagnostic step | | --- | --- | --- | --- | | P0171 | System too lean, Bank 1 | Intake leak, MAF sensor, fuel pressure, oxygen sensor feedback | Check fuel trims, intake leaks, and MAF readings | | P0300 | Random or multiple cylinder misfire | Spark plugs, coils, injectors, compression, fuel delivery | Check misfire counters and cylinder-specific codes | | P0420 | Catalyst efficiency below threshold | Catalytic converter, oxygen sensors, exhaust leaks, misfires | Check upstream faults before replacing the converter | | P0128 | Coolant temperature below regulating range | Thermostat, coolant temperature sensor, cooling system | Compare coolant temperature rise with expected warm-up behavior | | P0401 | EGR flow insufficient | EGR valve, passages, vacuum/electrical control, differential pressure sensor | Check commanded vs measured EGR behavior | | P0455 | EVAP system large leak | Fuel cap, EVAP hoses, purge valve, vent valve, charcoal canister | Inspect cap and perform EVAP leak test if needed | ## Reading fault codes with AutoPi AutoPi devices can be used to read and log diagnostic trouble codes from supported vehicles. In fleet and development work, the main benefit is not only reading a code once. The benefit is having fault-code history, timestamps, position data and related vehicle data available over time. A typical diagnostic workflow with AutoPi can include: - **Fault-code readout:** Read active and stored DTCs from the vehicle where it is supported. - **Time and location context:** Store when and where the fault was detected. - **Live data comparison:** Compare DTCs with live values like voltage, temperature, speed, RPM and other supported signals. - **Fleet overview:** Monitor repeated fault patterns across vehicles instead of handling each vehicle alone. The Fleet Management view of vehicles in AutoPi Cloud. [Contact us](https://www.autopi.io/sales-inquiry/) to learn more about AutoPi Cloud. This is useful when a fault only happens under specific conditions. A short workshop scan may not show enough context. Logged data can show what happened before and after the fault was detected. ## Interpreting data in AutoPi Cloud AutoPi Cloud can be used to view vehicle data, fault-code history, device status, positions and selected telemetry. For diagnostics, this helps connect the fault code with the operating conditions. For example, a voltage-related code is easier to understand when you can also see supply voltage history. A misfire code is easier to investigate when it can be compared with RPM, load, temperature and when the fault happened. The cloud view is especially useful for fleets because it gives one place to compare vehicles, recurring codes and fault frequency. This makes it easier to prioritize service work and identify repeated patterns. The Fleet Management overview in AutoPi Cloud. [Try the AutoPi Cloud demo](https://demo.autopi.io/). ## Resetting fault codes with AutoPi Cloud Fault codes can be cleared after the issue has been inspected or repaired. Clearing the code tells the diagnostic system to remove the stored code. But it does not fix the fault that caused it. In practice, clearing codes should be handled carefully: - **Read the code first:** Store or document the DTC, freeze-frame data and relevant live data. - **Check the root cause:** Inspect the system indicated by the code before clearing it. - **Clear only when appropriate:** Clear the code after repair, testing or controlled diagnostic work. - **Verify that it does not return:** Run the vehicle and check if the same code comes back. Professional help is recommended for recurring faults, flashing Check Engine Light, misfires, catalytic converter issues, transmission-related faults, high-voltage EV faults, braking systems, airbags and safety-related systems. The Diagnostic Trouble Codes view in AutoPi Cloud. [Try the AutoPi Cloud demo](https://demo.autopi.io/). ## Using fault-code data in fleet maintenance For a single vehicle, a fault code helps with troubleshooting. For a fleet, fault-code data becomes more useful when it is collected consistently across many vehicles. With AutoPi, fleet operators can use DTC history together with location, time and vehicle telemetry to see recurring issues. This can help prioritize service, identify vehicles with repeated faults and reduce unnecessary manual checks. Useful fleet-level checks include: - **Repeated faults:** See which vehicles report the same fault again and again. - **Operating conditions:** Check if faults happen under specific load, temperature, speed or location. - **Signals before the fault:** See if voltage, temperature or load changes appear before the fault. - **Repair verification:** Check if a repair actually prevents the code from returning. This is more reliable than judging maintenance needs from a single scan. Historical data gives context. ## Vehicle monitoring with AutoPi AutoPi can be used for more than Check Engine Light diagnostics. Depending on the vehicle and configuration, the platform can collect OBD2, CAN, CAN FD, J1939, GPS and device status data. This supports a more structured maintenance workflow. Instead of waiting for a driver to report a warning light, the system can log faults and make them visible in the cloud. This is useful for workshops, fleet managers, engineering teams and connected vehicle projects. Common use cases include diagnostics, vehicle tracking, data logging, utilization monitoring, service planning and integration with existing backend systems. [ Questions? Get in touch ](https://www.autopi.io/sales-inquiry/) ## FAQs about Check Engine Light Read the fault code first with an OBD2 scanner or diagnostic tool. After the fault has been repaired or checked, the code can be cleared with the tool. If the fault is still present, the light will normally return after one or more drive cycles. Avoid clearing codes before saving the information, because the code and freeze-frame data can help with diagnosis. It usually appears as an engine-shaped icon or as “Check Engine” text. The exact symbol depends on the vehicle. The ECU has detected a fault or abnormal condition. Common causes include EVAP leaks, misfires, oxygen sensor faults, fuel trim problems, catalytic converter efficiency faults, and sensor or wiring issues. Read the DTC to know where to start. A flashing Check Engine Light often indicates a severe misfire. Continued driving can damage the catalytic converter. Reduce load, avoid hard acceleration and get the vehicle checked as soon as possible. If the light is steady and the vehicle behaves normally, you can often drive carefully to a place where the code can be read. If the light is flashing, the vehicle runs poorly, or there are warnings for oil pressure, temperature, brakes, battery or high-voltage systems, stop and seek professional help. On some vehicles, disconnecting the battery may clear the light temporary, but it can also reset learned values, radio settings or other systems. A scanner or diagnostic tool is the better method because it lets you read and save the code before clearing it. This article gives general guidance. Always check the vehicle manual, service information or a qualified mechanic for vehicle-specific diagnostics and repair. [ ](https://www.autopi.io/hardware/autopi-canfd-pro/) [ Discover AutoPi CAN FD Pro ](https://www.autopi.io/hardware/autopi-canfd-pro/) [ ← Back to all posts ](https://www.autopi.io/blog/) // hardware ## Capture data with CAN-FD Pro Evaluate CAN, CAN FD and J1939 logging. Confirm vehicle access, connectors and signal decoding before deployment. [Explore CAN-FD Pro →](https://www.autopi.io/hardware/autopi-canfd-pro/) ###### Hardware ## Choose your evaluation device Start with the hardware your project needs. Software and connectivity terms differ by device and purchase option. Prices below exclude VAT, duties, shipping and optional accessories. CAN-FD Pro In stock · ships tomorrow ### Dual CAN-FD data logger High-throughput raw frame capture on two independent CAN-FD channels. On-device DBC decoding, ASAM MDF4 output, J1939 native support and hardware-encrypted data signing - all on a Raspberry Pi CM4. → 2× CAN-FD · >3,000 fps/channel · 5 Mbps → 4 GB LPDDR4 · 32 GB eMMC + USB expansion → NXP SE051 · Tailscale SSH · Docker · S3 sync → 12–35 V DC · J1939 · OBD-II · 4G/LTE + GPS From EUR 599. AutoPi software included without a subscription. Cellular SIM and data purchased separately. [Order CAN-FD Pro](https://shop.autopi.io/products/autopi-can-fd-pro) [Learn more](https://www.autopi.io/hardware/autopi-canfd-pro/) Mini In stock · ships tomorrow ### Fleet telematics & OBD-II OBD-II and supported EV telemetry with GPS and built-in 4G/LTE. Check your exact vehicle, required signals and regional variant; installation and configuration needs vary. → OBD-II / K-Line · EV parameters · GPS tracking → 4G/LTE Cat 1 · BLE · internal antennas → −40°C to +85°C · CE/UKCA/E-Mark/RoHS → 10–30 V DC · compact · fleet-ready out of box EUR 129 + EUR 6/month for SIM and Cloud, with a 12-month minimum. First-year minimum: EUR 201. Includes 100 MB/month. [Order Mini](https://shop.autopi.io/products/autopi-mini) [Learn more](https://www.autopi.io/hardware/autopi-mini/) TMU CM4 In stock · ships tomorrow ### Edge compute platform Raspberry Pi CM4-based telematics unit for custom software stacks. Run Docker containers, Python services and CAN-FD logging on the same device - managed remotely via AutoPi Cloud. → BCM2711 CM4 · 1 GB LPDDR4 · 8 GB eMMC → 2x CAN or CAN-FD variant · Docker · Python · SocketCAN → NXP SE051 · Tailscale · WiFi · BT 5.0 · GPS → 12–35 V DC · 4G/LTE Cat 4 · IP67 option From EUR 235 with the SIM and Cloud subscription option: EUR 6/month, 12-month minimum. Also available without a subscription; select your interface and payment option in the shop for the price. [Order TMU CM4](https://shop.autopi.io/products/autopi-telematics-unit-cm4-4g-lte-edition) [Learn more](https://www.autopi.io/hardware/autopi-tmu-cm4/) [Compare all devices →](https://www.autopi.io/hardware/compare/) // related ## More posts you'll like [ View all → ](https://www.autopi.io/blog/) Raspberry Pi DIY #### Build a Raspberry Pi Touchscreen Car Computer (Step-by-Step) Assemble a Pi-based carputer with touchscreen, power, mounting and software setup. Parts list, wiring tips and configuration for a reliable in-car sys ... [ Learn More → ](https://www.autopi.io/blog/build-a-raspberry-pi-touch-screen-car-computer/) Guides #### J1939 Explained (2025): PGNs, SPNs & Heavy-Duty Diagnostics Learn J1939 basics‚ PGNs, SPNs, addressing and tools for heavy-duty trucks and machinery. Examples and troubleshooting tips included. [ Learn More → ](https://www.autopi.io/blog/j1939-explained/) Guides #### CAN Bus Explained (2025): Frames, Arbitration & Tools Understand identifiers, arbitration, error handling and analyzers used in automotive systems. Practical examples and diagrams. [ Learn More → ](https://www.autopi.io/blog/can-bus-explained/) // contact ## Still have questions? Get in touch with us - we're ready to answer any and all questions. // send a message We aim to reply within one business day. Or email [sales@autopi.io](mailto:sales@autopi.io).