// blog # Automotive DTCs: Codes, Causes & Fix Paths (Guide) Understand P/B/C/U code families, common causes and repair steps - with examples and prevention tips for fleets and DIYers. Updated 14 Aug, 2025 [← All posts](https://www.autopi.io/blog/) ## Need a simple, practical intro to automotive diagnostic trouble codes? Automotive diagnostic trouble codes, also called DTCs, are used to identify vehicle faults. When a warning symbol appears on the dashboard, the vehicle diagnostic system has normally detected a problem and stored a fault code. The code gives a starting point for troubleshooting. It does not always tell exactly which part is broken, but it helps narrow down where to look first. For fleet managers, technicians and vehicle owners, understanding DTCs can help reduce downtime, improve safety and plan maintenance better. ## What Is Automotive Diagnostic Trouble Codes? Diagnostic trouble codes are five-character alphanumeric codes used by the vehicle OBD system. They describe a fault detected by the vehicle electronics. The driver may only see a dashboard warning light, but behind that warning there is often a stored code that can be read with a diagnostic tool or telematics device. The first letter in the code describes the main vehicle area: - **P (Powertrain):** Relates to engine, transmission and emissions systems. - **B (Body):** Covers comfort, safety and auxiliary systems like airbags, seat belts and body electronics. - **C (Chassis):** Covers braking, steering and suspension systems. - **U (Network and vehicle integration):** Covers communication errors between vehicle modules, such as [CAN bus](https://www.autopi.io/software-platform/canbus-management/) faults. The second character tells if the code is a generic OBD-II code or a manufacturer-specific code. The third character points to the vehicle system involved in the fault. Common third-character categories include: - **1:** Fuel and air metering. - **2:** Fuel and air metering, normally injector circuit related. - **3:** Ignition system or misfire. - **4:** Emissions control. - **5:** Idle control system and vehicle speed control. - **6:** Computer auxiliary outputs. - **7, 8 and 9:** Gearbox and transmission faults. - **A, B and C:** Hybrid propulsion faults. The picture below shows how the code is built: The OBD system logs DTCs automatically when it detects an issue. For example, if there is a misfire on cylinder 1 in an internal combustion engine, the vehicle can store **P0301 (Cylinder 1 Misfire Detected)**. This can be caused by a spark plug, ignition coil, injector issue, compression issue or another problem in that cylinder. The code does not repair the vehicle. But it gives a good place to start the diagnosis. ## Why Are DTCs Important? When a [DTC code](https://www.autopi.io/glossary/diagnostic-trouble-codes/) is active, it normally means the vehicle has detected a fault that needs attention. For fleets, this is important because one vehicle with a fault can affect uptime, safety, fuel use and compliance. An example is **P0420 (Catalyst System Efficiency Below Threshold - Bank 1)**. This code indicates that the catalytic converter system may not be working as expected. If it is ignored, the vehicle can have higher emissions and may fail an emissions test. With the [AutoPi Fleet Management Platform](https://www.autopi.io/business-solutions/fleet-management-system/), it is possible to monitor [diagnostics in real-time](https://www.autopi.io/software-platform/fleet-management/) across a fleet. This makes it easier to see which vehicles are in a fault state and plan maintenance without inspecting every vehicle manually. [ Learn More About Fleet Management ](https://www.autopi.io/business-solutions/fleet-management-system/) ## How AutoPi Helps Manage Automotive Diagnostic Trouble Codes AutoPi provides [devices](https://www.autopi.io/hardware/compare/) and [cloud](https://www.autopi.io/software-platform/cloud-management/) software for collecting vehicle and heavy-machinery telematics data. Part of this is reading, interpreting and analyzing diagnostic trouble codes. AutoPi devices connect to the vehicle and can record active trouble codes. Through the cloud platform, the codes can be reviewed, interpreted and used to decide the next maintenance action. Example codes are: - **P0171 (System Too Lean - Bank 1):** Indicates an air-fuel imbalance, which can affect engine performance and fuel use. - **P0300 (Multiple Cylinder Misfire Detected):** Indicates random or multiple cylinder misfires, often related to ignition, fuel or air-fuel problems. With the AutoPi platform and devices, it is possible to understand the severity of specific trouble codes from a more human readable explanation. This can help the maintenance team decide what should be handled now and what can be planned for later. [ ](https://www.autopi.io/hardware/autopi-canfd-pro/) [ Discover Now! ](https://www.autopi.io/hardware/autopi-canfd-pro/) ## Common Automotive Diagnostic Trouble Codes and Their Implications Some DTCs appear often across many vehicles. Understanding these common codes helps fleet managers and technicians respond faster. - **P0455 (Evaporative Emission Control System Leak Detected - Gross Leak):** Often caused by a loose fuel cap or a leak in the EVAP system. It can affect emissions control and should be checked. - **P0302 (Cylinder 2 Misfire Detected):** Indicates a misfire on cylinder 2. It can be caused by spark plugs, coils, injectors or other engine problems. - **P0420 (Catalyst System Efficiency Below Threshold - Bank 1):** Indicates that the catalytic converter system may not be working efficiently. It can lead to higher emissions and should not be ignored. In a fleet context, these codes help prioritize maintenance. For example, an engine misfire such as **P0302** may require immediate action, while a small emissions leak such as **P0456** may be scheduled for later inspection. ## Managing Advanced DTCs As [vehicle technology](https://www.autopi.io/hardware/autopi-canfd-pro/) gets more advanced, DTCs also get more complex. Modern vehicles have more electronic systems, more control units and more communication between modules. This means some codes require deeper analysis than a simple engine fault code. If the vehicle detects abnormal behaviour, it stores the relevant DTC in the onboard computer. The code can then be read by diagnostic tools or telematics devices. - **C0035 (Left Front Wheel Speed Sensor Circuit):** Affects ABS performance and can create unsafe braking behaviour. - **B0028 (Passenger Side Airbag Deployment Loop Short to Ground):** A safety-critical fault that can affect airbag deployment. - **U0100 (Lost Communication with ECM/PCM “A”):** A network communication fault where the engine or [powertrain](https://www.autopi.io/glossary/powertrain/) control module cannot communicate correctly. Managing these advanced [automotive diagnostic trouble codes with AutoPi](https://www.autopi.io/hardware/autopi-canfd-pro/) helps fleet managers react faster and reduce the risk of small problems becoming larger failures. The platform can provide [real-time alerts](https://www.autopi.io/software-platform/cloud-management/) and context for each code, which helps reduce repair cost and avoid breakdowns. ## Proactive Fleet Management with AutoPi Fleet operators need to keep vehicles safe, efficient and compliant with regulations. Tracking DTCs with AutoPi helps catch problems earlier and reduce unplanned downtime. The system can monitor [vehicle health](https://www.autopi.io/software-platform/cloud-management/) and help sort, categorize and prioritize issues by severity. For example, **P0456 (Evaporative Emissions Leak - Small Leak)** may be monitored and planned for future maintenance. A **P0300 (Multiple Cylinder Misfire)** can require faster action because continued driving may lead to long-term engine damage. The practical value is that maintenance can be planned from data instead of only from driver reports or manual inspections. ## Conclusion Automotive diagnostic trouble codes are useful for understanding vehicle health. They help identify problems, prioritize repairs and reduce costly breakdowns. For a single vehicle, a DTC helps a technician know where to start. For a fleet, DTC data becomes more useful when it is collected continuously and shown in one platform. AutoPi takes this further by combining fault-code information with telematics, alerts and cloud access. Whether you are handling common DTCs like **P0301** or more complex codes like **U0100**, [AutoPi devices](https://www.autopi.io/hardware/autopi-canfd-pro/) can help collect the data needed to keep vehicles safe and operational. AutoPi for Developers Build, extend, and integrate using an open telematics platform Access device-level Linux, REST and MQTT interfaces, custom event rules, and full CAN/OBD data pipelines. Explore how AutoPi can be adapted to your own architecture and workflows. [ Explore Developer Options ](https://www.autopi.io/software-platform/device-management/) [ ← Back to all posts ](https://www.autopi.io/blog/) // hardware ## Choose hardware for your integration Compare Mini, CAN-FD Pro and TMU CM4 by vehicle data access, interfaces and custom software needs. [Compare devices →](https://www.autopi.io/hardware/compare/) ###### 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/) Guides #### How to Read CAN Bus Data: Extraction, Decoding & Tools Tap points, cables, sniffers and decoding workflows with examples. From raw frames to signal values using DBCs and analyzers. [ Learn More → ](https://www.autopi.io/blog/how-to-read-can-bus-data/) Guides #### CAN FD Adapter Explained: Specs, Compatibility & Top Picks (2025) Frame format, bit-rates, connectors and compatible vehicles - recommended hardware and software for CAN-FD projects. [ Learn More → ](https://www.autopi.io/blog/what-is-can-fd-adapter/) // 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).