Permanent Diagnostic Trouble Codes (DTCs), like P0128, are a common feature in vehicles manufactured after 2010. These codes indicate a past issue with your car’s system. While a stored permanent code with no active check engine light doesn’t necessarily signify a current problem, it’s crucial to understand their implications, especially regarding emissions testing.
What are Permanent DTCs and How Do They Work?
Permanent DTCs, such as P0128, are stored in your vehicle’s Engine Control Unit (ECU) after a fault is detected and subsequently repaired. Unlike temporary codes that clear after a certain number of drive cycles, permanent codes remain in the ECU’s memory to ensure the repair effectively addressed the issue. These codes are specifically designed for emissions compliance verification. Even if the check engine light is off, a permanent code like P0128 might still be present.
Passing Emissions Tests with a Permanent DTC
In many jurisdictions, like California, passing a smog check with a permanent DTC like P0128 is possible under specific conditions:
- No Active Check Engine Light: This indicates no current issues detected by the system.
- Completed Monitors: All emission-related monitors, except potentially the Evaporative System (EVAP), must have run and completed their diagnostic cycles.
- Drive Cycle Completion: The vehicle must be driven for a specific distance (e.g., 200 miles) and undergo a certain number of warm-up cycles (e.g., 15) since the last DTC clearing. This “200/15” rule allows the emissions system to thoroughly evaluate the repair’s effectiveness. Meeting these criteria signals to the testing equipment that the vehicle is operating correctly, even with a logged permanent code like P0128. The smog check machine is programmed to disregard the permanent code if the drive cycle requirements are met.
Understanding Warm-Up Cycles
A warm-up cycle involves the engine reaching normal operating temperature and then cooling down below a predetermined threshold. This process allows various sensors and systems to evaluate performance across different temperature ranges. Each vehicle has specific temperature parameters, but a common example is reaching 160 degrees Fahrenheit and then cooling down by at least 40 degrees.
Checking Drive Cycle Progress
Most OBD-II scanners provide data PIDs (Parameter IDs) that display the number of completed warm-up cycles and the distance driven since the last code clear. Consult your scanner’s documentation or “DataStream”/”Live Data” section to access this information. This allows you to monitor your progress toward meeting the 200/15 requirement before undergoing an emissions test.
Dealing with Incorrect Smog Check Refusals
If a smog check inspector refuses to test your vehicle solely due to a permanent code like P0128, inform them of the regulations regarding drive cycle completion and permanent DTCs. Insist on the test being conducted as the vehicle is presented. If necessary, file a complaint with the appropriate regulatory agency, like the Bureau of Automotive Repair (BAR) in California.
Conclusion
Permanent DTCs like P0128 record past issues and play a crucial role in emissions compliance verification. Understanding their function and the requirements for passing emissions tests, including completing the necessary drive cycle, can save you time and frustration. Don’t hesitate to assert your rights if faced with an inspector unfamiliar with the regulations surrounding permanent codes and emissions testing.
