When dealing with vehicle diagnostics, particularly concerning engine performance and emissions, the term “misfire” is frequently encountered. Modern vehicles equipped with OBD2 (On-Board Diagnostics II) systems have sophisticated monitors to detect and report misfires. One specific aspect that might come up is the “Mis-Fire Mon 200/1000 Pre-Test.” Let’s delve into what this refers to based on information from a Chrysler service manual.
The misfire monitor is a crucial part of the OBD2 system, designed to continuously check for engine misfires. It’s categorized as a “two-trip monitor,” meaning that a fault needs to be detected during two separate driving cycles to trigger the Malfunction Indicator Lamp (MIL), commonly known as the check engine light. A “trip” is defined as starting the vehicle and operating it under conditions that allow the monitor to run its diagnostics.
Within the misfire monitor, there are two primary counters or tests:
200 Revolution Counter: This counter is designed to detect severe misfires that can cause immediate damage to the catalytic converter. Catalytic converters are essential for reducing harmful emissions, and excessive misfires can lead to them overheating and failing.
1000 Revolution Counter: This counter is focused on detecting misfires that, while perhaps not immediately damaging to the catalyst, are still significant enough to increase vehicle emissions beyond acceptable limits – specifically, 1.5 times the Federal Test Procedure standards.
alt text: An OBD2 diagnostic port in a vehicle, used for connecting scan tools to read vehicle data and diagnose issues like OBD2 misfires.
For a misfire monitor to register a “good trip,” meaning it has successfully run and not detected a fault (or has confirmed a repair), certain conditions must be met. This is known as the “Similar Conditions Window.” The engine RPM must be within ±375 RPM, and the engine load must be within ±20% of the conditions present when a misfire fault was initially detected. If the engine operates for 1000 revolutions within this “Similar Conditions Window” without detecting a misfire, it counts as one “good trip.” Three successful “good trips” are typically required to turn off the MIL if it was illuminated due to a misfire.
Once the MIL has been turned off by the “Good Trip Counter,” the system then uses a “Warm-Up Cycle Counter.” Warm-up cycles are important for clearing Diagnostic Trouble Codes (DTCs) and Freeze Frame data. A “Warm-Up Cycle” is defined by several criteria:
- The engine must be started and run.
- The engine temperature must increase by 40°F (4.5°C) from the start.
- The engine coolant temperature must start below and rise above 160°F (71°C).
- A complete driving cycle, including engine start-up and shut-down, must occur without any new faults.
It typically takes 40 warm-up cycles for the Powertrain Control Module (PCM) to automatically erase a misfire DTC and its associated Freeze Frame data (some monitors, like the Fuel System Monitor and Misfire Monitor, may require 80 warm-up cycles).
Understanding the “Mis-Fire Mon 200/1000 Pre-Test” and the related conditions is crucial for both diagnosing misfires and verifying repairs. By monitoring these parameters, the OBD2 system ensures that misfires are detected and addressed, protecting both the catalytic converter and maintaining acceptable emission levels. When troubleshooting misfire issues, ensuring that no emission-related DTCs are present and that the OBD II monitors are running correctly is the first step towards accurate diagnosis and repair.
