Understanding Auto Obd2 Codes is crucial for modern vehicle maintenance and emissions testing. The On-Board Diagnostic (OBD) system in your car monitors various components for optimal performance and emissions control. This guide provides a detailed overview of OBD2 codes, pass/fail standards for emissions inspections, and common issues encountered during OBD testing.
Understanding the Basics of OBD2 and Auto Diagnostic Codes
The OBD-II system is designed to be a standardized system that provides vehicle self-diagnosis and reporting capabilities. When your vehicle detects a problem that could affect emissions, it generates a Diagnostic Trouble Code (DTC), commonly known as an auto OBD2 code, and may illuminate the Malfunction Indicator Lamp (MIL), often referred to as the “Check Engine” light.
For smog check inspections and general auto repair, understanding these codes is essential. The OBD test portion of a smog check evaluates several aspects:
- Readiness Monitors: These are self-tests that the vehicle’s computer runs to ensure emission control systems are functioning correctly.
- Malfunction Indicator Lamp (MIL): The infamous “Check Engine” light. Its behavior (on or off in different key positions) is a key indicator.
- Communication: The ability of the inspection equipment to communicate with the vehicle’s computer to retrieve data.
- Permanent Diagnostic Trouble Codes (PDTCs): Codes that cannot be cleared by simply resetting the system and indicate a confirmed fault.
- Software Integrity: Checking for illegally modified vehicle software.
Let’s delve into the pass/fail standards for each of these areas in an OBD test.
Pass/Fail Standards for OBD2 Emissions Tests
The criteria for passing or failing the OBD test during a smog check are based on several factors. Here’s a breakdown:
Readiness Monitor Standards
Readiness monitors confirm that your vehicle’s emission control systems are ready for testing. Vehicles need to complete these self-tests. However, some incomplete monitors are allowed depending on the vehicle’s model year and fuel type.
| Model Year | Fuel Type | Allowed Incomplete Monitors |
|---|---|---|
| 1996-1999 | Gas (including gasoline, propane, natural gas) | Any one |
| 2000 and newer | Gas (including gasoline, propane, natural gas) | Evaporative system only |
| 1998-2006 | Diesel | Zero |
| 2007 and newer | Diesel | Particulate Filter and NMHC |
It’s important to note that continuous monitors like misfire, fuel, and comprehensive component monitors are typically ignored during OBD tests on gasoline vehicles.
Malfunction Indicator Lamp (MIL) Standards
The MIL, or “Check Engine” light, has specific pass/fail criteria:
| Condition | Standard | Result |
|---|---|---|
| Key On Engine Off (KOEO) | MIL illuminates | Pass |
| Engine Running (KOER) | MIL turns off | Pass |
| Key On Engine Off (KOEO) | MIL does not illuminate | Fail |
| Engine Running (KOER) | MIL illuminates | Fail |
Essentially, the MIL should turn on when the key is turned to the “on” position before starting the engine, and then turn off once the engine is running if there are no active emission faults.
Communication Standards
Successful communication between the vehicle and the testing equipment is vital. Failure to communicate can be due to various issues:
| Test Type | Standard |
|---|---|
| BAR-97 | Vehicle communicates with BAR-97 equipment. |
| OIS | Vehicle communicates with OIS and transmits required data (Mode $01 PID $00, Mode $01 PID $01). |
Communication failures can stem from a damaged OBD connector, wiring problems, aftermarket stereos interfering with the system, or even incorrect computer responses from the vehicle.
Permanent Diagnostic Trouble Code (PDTC) Standards
PDTCs are a critical aspect of modern OBD testing. Introduced in 2010 and newer vehicles, PDTCs are like regular DTCs but cannot be erased by normal means. They clear only when the vehicle itself verifies the issue is resolved.
| Model Year | Standard | Result |
|---|---|---|
| 1996-2009 | Not applicable | N/A |
| 2010 and newer | No PDTCs present | Fail if PDTC is present |
Vehicles will fail the OBD test if any PDTCs are present, regardless of whether the MIL is currently on or off. There are some exceptions for vehicles that have completed numerous warm-up cycles and driving miles since the last OBD data clear, and for specific vehicles known to have issues clearing PDTCs (more on this later).
Modified Software Standards
Tampering with vehicle software to bypass emissions controls is a serious issue. Smog checks now include a check for modified software.
| Condition | Standard | Result |
|---|---|---|
| Software Configuration | OEM-approved or CARB-approved | Pass |
| Software Configuration | Illegally modified | Fail – “Modified Software” |
Vehicles with illegally modified software will fail the smog check and must be restored to an approved software configuration before re-testing.
Common OBD2 Test Issues and Vehicle-Specific Information
Certain vehicles are known to have specific issues that can affect OBD testing, leading to communication problems, incomplete monitors, or false MIL indications. These issues are often due to design anomalies or software glitches. Here are some examples categorized by the type of issue:
Vehicles with Communication Issues:
- Audi (2000-2004) A4 & A6: May fail due to no OBD communication, often caused by a broken ground wire in the ABS module. Repairing the ABS module ground wire is the remedy.
- BMW (2001-2003) Z3, Z8 & M5: Can fail for no communication when the ignition is switched off/on. A software update recall addresses this.
- Ford Escape, Bronco Sport, E-Series, Corsair (2020-2023): These models may have communication issues with BAR-OIS testing equipment due to gateway module software. A dealer software update is usually required.
- Ford Explorer (2025+): Newer models may use OBDonUDS communication protocols, which older DAD 1.0 test units cannot handle, leading to communication failure.
Vehicles with Readiness Monitor Issues:
- BMW 3 Series, 5 Series, Z3 (1998-2000): Various models in this era may have trouble setting the oxygen sensor readiness monitor due to DME software errors. A DME software reprogram is the fix.
- Chevrolet S-10, GMC Sonoma (2000-2002) 2.2L E85: Flex-fuel models can have monitors disabled when running on higher ethanol blends. OIS systems are programmed to ignore readiness monitors on these specific vehicles.
- Ford Focus (2003): Monitors may be difficult to complete. A PCM reflash might be necessary.
- Hyundai Sonata (2002), Suzuki Aerio (2001), Forenza (2004), Toyota (2000-2006 all models): These vehicles are generally known to have monitors that are difficult to set, often requiring specific drive cycles to complete.
Vehicles with Permanent Diagnostic Trouble Code (PDTC) Issues:
Some vehicles are known to store PDTCs that are difficult to clear, even after the underlying problem is resolved. In many cases, software reflashes are available to address these issues. Examples include:
- Chrysler, Dodge, Jeep, Ram (2010-2015): Many models across these brands from this period can have PDTCs that won’t clear, often resolved by recalls like Recall V51 (software reflash). Specific models include Chrysler 200, 300, Town & Country, Dodge Avenger, Caliber, Challenger, Charger, Grand Caravan, Dakota, Dart, Durango, Journey, Nitro, Jeep Cherokee, Commander, Grand Cherokee, Liberty, Patriot/Compass, Wrangler, and Ram 1500, 2500, 3500, ProMaster.
- Ford Mustang (2011-2014): May fail for PDTC P0315, requiring a visit to a Ford dealer for Customer Satisfaction Program 17N01.
- Freightliner, Mercedes-Benz Sprinter (2014-2015): Certain Sprinter models may have PDTCs (P202A, P203A, etc.) related to aftertreatment systems that are ignored by BAR-OIS during testing.
- Audi, Volkswagen Touareg (2010-2012/2013) Diesel: Certain diesel models may have PDTCs (P0087, P0088, etc.) related to fuel pressure that are also ignored by BAR-OIS.
Vehicles with False MIL Illumination Issues:
- Dodge Ram Truck (2003) 5.9L Diesel: The MIL light might be commanded on, but the check engine light is not actually illuminated, and no DTCs are stored. This is a design anomaly addressed by recall campaign #K01 (ECM reflash).
- Mazda 6 (2003-2005): These vehicles may exhibit false MIL illumination and U-codes during OBDII testing if the scan tool is connected or disconnected with the ignition on. The solution is to reset monitors and ensure tool connection/disconnection is done only with the key off.
Conclusion: Navigating Auto OBD2 Codes and Vehicle Diagnostics
Understanding auto OBD2 codes and the intricacies of OBD testing is vital for both vehicle owners and technicians. While the OBD system is designed to be standardized, vehicle-specific issues and software anomalies can complicate diagnostics and emissions testing.
This guide provides a comprehensive overview of OBD2 test standards and highlights many vehicles known to have OBD-related problems. When encountering OBD test failures or unusual auto OBD2 codes, consulting vehicle-specific service information and technical service bulletins (TSBs) is always recommended. For complex issues, seeking professional diagnosis and repair from a qualified mechanic is crucial to ensure your vehicle meets emissions standards and operates efficiently.
