For anyone involved in vehicle maintenance and repair, grasping the intricacies of the On-Board Diagnostics II (OBD2) system is crucial. Among the many parameters monitored by OBD2, the “Warm Ups Obd2” concept plays a significant role, especially in emissions testing and diagnostic procedures. This article delves into the warm-up cycle, its related components, and its importance in understanding your vehicle’s health and diagnostic processes.
After each engine startup, your vehicle initiates a warm-up cycle. This cycle is defined by the engine coolant temperature (ECT) increasing by a minimum of 40°F (22°C) and reaching at least 160°F (71°C). The Engine Control Module (ECM) diligently tracks these warm-up cycles. Accumulating 40 such cycles triggers the ECM to clear historical Diagnostic Trouble Codes (DTCs) and Freeze Frame Data (FFD). It’s important to note that this action does not reset readiness flags or clear Permanent Diagnostic Trouble Codes (PDTCs), nor is it a prerequisite for running monitor tests.
The warm-up cycle counter provides valuable insight into the vehicle’s recent diagnostic history. A low counter number, like 7 as shown in some scan tools, suggests that the vehicle’s memory was recently cleared. Conversely, the counter will cap at 255, indicating a significant period since the last memory reset.
Cold Soak: Setting the Stage for a Cold Start
Before a cold start can be registered, specific conditions must be met. The engine coolant temperature (ECT) and intake air temperature (IAT) sensors need to register temperatures within approximately 10°F of each other and both must be below 122°F (50°C) at startup. These cold start conditions are sometimes essential for initiating monitor tests for critical emission control systems. These systems include the air-fuel ratio sensor heater(s) (AFRS), oxygen sensor heater(s) (HO2S), secondary air injection (AIR), and the evaporative emission system (EVAP).
Enable Criteria: Conditions for Diagnostic Tests
Enable Criteria, also known as Enable Conditions, are specific engine operating parameters and driving conditions that must be met for a monitor test to run. Each Diagnostic Trouble Code (DTC) is associated with unique enabling conditions. When these criteria are satisfied, the ECM will execute the corresponding monitor test(s). If the enable criteria are not met, the test will not be performed. A failure during a monitor test can result in a pending or confirmed DTC being set.
Consider the Secondary Parameters displayed on a scan tool. For a P0135 DTC, it might be necessary to have no active DTCs present for the test to commence. Secondary parameters encompass these enabling conditions. P0135 is classified as a Type B DTC, meaning the monitor test must fail on two consecutive driving trips to illuminate the Malfunction Indicator Lamp (MIL).
Trip: A Single Test Execution Sequence
In OBD2 terminology, a “trip” signifies a complete diagnostic test sequence. A trip begins with a key-on engine-off (KOEO) or key-on engine running (KOER) condition, proceeds through the fulfillment of the enable criteria for a specific monitor test, and concludes with a key-off event. The trip is considered finished when the scan tool loses communication with the ECM.
When the Malfunction Indicator Lamp (MIL) is activated, the ECM/PCM is designed to deactivate it after three consecutive trips where the same fault is not detected. For example, if a vehicle initially sets a DTC P0420, the MIL will turn off if, over three subsequent trips, the driver operates the vehicle in conditions that meet the enabling criteria for P0420, and the ECM/PCM does not detect the fault again. In such cases, the DTC will be relegated to historical code status but will no longer trigger the MIL.
Combining Enable Criteria and a Trip in Diagnostics
The process of combining enabling criteria with a trip is fundamental to OBD2 diagnostics. During a test drive, for example, the enabling criteria for the Catalyst monitor test might be met and then interrupted multiple times due to fluctuating driving conditions. These variations can temporarily suspend the monitor test. However, once the conditions are consistently met for the required duration, the monitor test will register as “Done This Trip.”
It’s crucial to understand that this focused approach on a single readiness flag and its associated monitor test differs from a complete drive cycle. The data shown in the example was captured using a Chrysler DRBIII scan tool.
Drive Cycle: Comprehensive Readiness Testing
A drive cycle encompasses both the Enable Criteria and a Trip. To successfully complete an entire drive cycle, the enable criteria for all relevant monitor tests must be met within a single key cycle. Ideally, this process should result in all readiness flags being flipped to “Complete.” A drive cycle is considered complete, and the trip concludes when the ignition key is turned off, and the scan tool disconnects from the vehicle’s communication network.
Achieving complete readiness flags in a single drive cycle is often unrealistic for everyday drivers or technicians. Numerous variables, such as traffic conditions, road types, and freeway access, influence the process. Typically, drive cycles necessitate a combination of part-throttle driving and steady cruising at highway speeds.
For in-depth information on drive cycles, you can refer to resources specifically dedicated to the OBDII Drive Cycle.
Readiness Flag: Reporting Emission System Test Completion
A readiness flag acts as a status indicator, signaling that a specific emission-related system has undergone and completed its self-testing. Each readiness flag can represent the outcome of one or more individual monitor tests. State emissions inspection programs often rely on these readiness flags as part of their official assessment. A minimum number of readiness flags must be marked “Completed” for a vehicle to pass an emissions inspection.
Readiness Flag Status: Complete, Not Complete, and Implications
The readiness flag status provides information on whether the ECM/PCM has finished testing particular monitored emission systems. It’s important to understand that this status does not indicate whether the system passed or failed the test. If the monitor test required to set a readiness flag to “complete” has not yet finished, the scan tool will display a status of “Not Complete.” Conversely, if the necessary monitor test has been successfully executed, the readiness flag status will show “Complete.” A “Complete” status signifies that the ECM/PCM has tested the system at least once since the last memory reset.
Incomplete, Not Done, Not Ready: Pre-CAN Systems
In vehicles manufactured before the widespread adoption of Controller Area Network (CAN) in 2008 (Pre-CAN), a status of “Incomplete,” “Not Done,” or “Not Ready” typically indicated that one or more required readiness flag monitor tests had either not been run or had failed since the last MODE $04 Diagnostic Clear was performed. Before CAN, the PCM was not mandated to switch to “Complete,” “Done,” or “Ready” if a readiness flag monitor test failed.
Complete, Done, Ready: Post-CAN Systems
With the introduction of CAN and in subsequent systems, the interpretation changed. “Complete,” “Done,” or “Ready” signifies that since the last MODE $04 Diagnostic Clear, all “Complete” readiness flag monitor tests have been executed at least once. Post-CAN, the readiness flag status must transition to “Complete,” “Done,” or “Ready” after testing completion, irrespective of whether the monitor tests passed or failed.
Non-Readiness Mode: System Operation Without Active Testing
Non-Readiness Mode describes a state during normal vehicle operation where the enable criteria needed to initiate a monitor test and set a readiness flag to “complete” are not being met. In this mode, systems like the EGR system might be functioning as designed – the EGR Command is active, indicating the solenoid is receiving an “ON” signal, and the EGR diaphragm is moving, confirming vacuum passage. The MAP sensor might also show intake manifold pressure changes when the valve opens. However, in non-readiness mode, the system is not being actively tested for flow or performance.
Readiness Mode: System Operation Under Active Testing
Readiness Mode, in contrast, occurs during regular operation when the enable criteria to run monitor test(s) and set a readiness flag to “complete” are met. A key differentiator between the two modes is often the presence of conditions like “Fuel Cut Decel.” During deceleration, for instance, the EGR solenoid might be commanded “ON,” the EGR position sensor moves due to intake manifold vacuum, and the MAP sensor reports these changes to the ECM. When the ECM completes system testing under these conditions, the readiness flag will be flipped to “Complete.” If a DTC is detected, the system fails; otherwise, it passes.
Monitor Test: System-Specific Diagnostic Evaluation
A monitor test is a system-specific diagnostic procedure designed to evaluate the performance of a component or system and potentially set a Diagnostic Trouble Code (DTC). One or more monitor tests are required to complete the testing for a component or system and flip the associated readiness flag to “Complete.” When a required monitor test runs and passes, the corresponding readiness flag will be set to “Complete.” If a DTC is present during the monitor test, the system is considered to have failed; if no DTC is set, the system passes.
Continuous Monitor Tests: Real-Time System Checks
Continuous monitor tests are executed by the ECM in real-time, constantly monitoring systems like misfires, fuel systems, and comprehensive components whenever their specific enable criteria are met. A continuous monitor test can run multiple times within a single trip and can even pass and fail during the same trip. Importantly, the readiness flag status for misfire, fuel system, and comprehensive components are typically not used during state emissions inspections for gasoline vehicles, although they may be relevant for diesel vehicles in some states.
Non-Continuous Monitor Tests: Periodic System Evaluations
Non-continuous monitor tests are designed to verify the performance of systems that do not require constant monitoring. These include the catalytic converter, oxygen/air-fuel sensor(s) and their heaters, EGR/VVT, EVAP, and secondary air injection. When the enable criteria for a specific non-continuous monitor test are met, the ECM will test the system at least once per trip if it passes. If a failing test result is indicated, the ECM may run the monitor test multiple times in the same trip to confirm the fault. The status of Non-Continuous Readiness Flags is displayed in MODE $01 of OBD2 data. State Emissions Inspection programs commonly use these readiness flag statuses as crucial criteria for determining pass/fail outcomes.
Malfunction Indicator Lamp (MIL): The Emission Warning Signal
The Malfunction Indicator Lamp (MIL), commonly known as the “check engine light,” is located on the vehicle’s dashboard. It is designed to illuminate when the ECM predicts that tailpipe emissions are exceeding 1.5 times the Federal Test Procedure standards. When the MIL turns on, a corresponding DTC and Freeze Frame data should be stored in the vehicle’s memory, aiding in diagnosis. If the MIL flashes while the engine is running, it typically indicates a misfire or fuel control system problem that is severe enough to potentially damage the catalytic converter.
In vehicles manufactured after 1999, the PCM has the capability to turn off a fuel injector to protect the catalytic converter in severe misfire conditions. Once an injector is disabled, the MIL will remain on steadily because, although the immediate threat to the catalytic converter is reduced, the underlying DTC is still present.
A MIL bulb check is a standard procedure performed during key-on engine-off (KOEO) for inspection and maintenance programs. During this check, the MIL should illuminate for approximately 5 seconds and then turn off. In vehicles manufactured after 2005, some manufacturers have implemented a feature where the MIL will flash during KOEO if one or more readiness flags remain “Incomplete.” However, this flashing MIL does not specify which readiness flag(s) are incomplete or how many. Importantly, even if the EVAP readiness flag is incomplete, the vehicle might still pass a state inspection test in many jurisdictions, as EVAP readiness is not always a mandatory pass criterion. If the MIL goes out after the 5-second bulb check and does not illuminate again, it generally indicates that either all readiness flags are set to “complete,” or the vehicle manufacturer does not support the flashing MIL feature for incomplete readiness flags. Unfortunately, the flashing MIL to indicate readiness flag status was not universally adopted, leading to variations in manufacturer implementation.
MIL Flashes Five Times After the Bulb Check:
Note: If the MIL flashes, the vehicle could still be ready for a state emissions inspection if the EVAP Readiness Flag is the only one that is not complete. No state emissions program requires EVAP to be Complete.
MIL Goes OFF After the Bulb Check:
Either the vehicle does not support this test, or all Readiness Flags are Complete and ready for a state emissions inspection.
OBD2 Communication Protocols: Evolution from Pre-CAN to UDS
Pre-Controller Area Network (Pre-CAN)
Pre-Controller Area Network systems were prevalent in vehicles from model year 1996 through 2007. Communication protocols utilized during this era included SAE J1850 PWM, SAE J1850 VPM, ISO 9141, and ISO Keyword Protocol 2000 (ISO 14230).
Controller Area Network (CAN)
Controller Area Network (CAN) protocol, specifically CAN ISO 15765, became the standard for powertrain module communication through DLC pins 6 and 14. Operating at 500kps, CAN ISO 15765 is mandatory on all model year 2008 and newer vehicles.
CAN Phase-in
The transition to CAN “C” protocol occurred gradually from model years 2005 through 2007. Identifying precisely which vehicles during these years were equipped with CAN protocol can be challenging. It is crucial to be aware that vehicles from this phase-in period may incorporate CAN “C” and communicate with scan tools via DLC pins 6 and 14.
Onboard Diagnostics on Unified Diagnostic Services (OBDonUDS)
The latest evolution in OBD2 communication is Onboard Diagnostics on Unified Diagnostic Services (OBDonUDS). OBDonUDS employs a new communication protocol, ISO 14229, while still utilizing CAN communication (pins 6 and 14) through the standard OBD-II connector (SAE J1962). The primary difference lies in the CAN message structure, which includes two additional layers of DTC-specific information, enhancing diagnostic capabilities and data resolution. The first vehicles implementing OBDonUDS, such as the 2025 Ford models, have recently been released. For more detailed information, refer to resources on Unified Diagnostic Services.
Understanding “warm ups obd2” and related concepts like drive cycles, readiness flags, and monitor tests is fundamental for effective vehicle diagnostics and ensuring compliance with emission standards. This knowledge empowers technicians and knowledgeable vehicle owners to accurately interpret OBD2 data, troubleshoot issues, and maintain vehicle health efficiently.
