Verifying a successful car repair goes beyond simply clearing diagnostic trouble codes (DTCs) and hoping the malfunction indicator light (MIL) stays off. For expert auto technicians, the crucial next step is a thorough road test to confirm the effectiveness of the repair. But the methodology of this road test is paramount. Is a quick jaunt around the block sufficient? Should you execute a complete generic OBD II drive cycle until all Inspection and Maintenance (I&M) readiness monitors are set? Or is a manufacturer-specific DTC drive cycle the most appropriate approach?
The answer to these questions holds significant weight, potentially marking the difference between a job well done and a frustrating comeback. A vehicle’s powertrain control module (PCM) is the ultimate authority, and ensuring its satisfaction is key to a successful repair. A meticulously performed road test will not only validate the immediate fix but also uncover any underlying issues masked by the initial fault.
For those familiar with OBD II systems, terms like “readiness monitors” and “drive cycle” are likely commonplace. However, for technicians less acquainted, let’s delve into a review.
Understanding the OBDII Drive Cycle
So, what exactly is an OBDII drive cycle? Essentially, it’s a predefined set of driving conditions engineered to activate the supported OBD II readiness monitors. These conditions are designed to mimic typical driving scenarios and are tailored to the specific vehicle’s configuration. The PCM’s primary objective during a drive cycle is to assess whether the emissions-related components are functioning correctly. Subsequently, the PCM utilizes these components to conduct fundamental tests, determining if the OBD II system can detect instances where tailpipe emissions surpass 1.5 times the Federal Test Procedure (FTP) emissions standard. Ideally, this process would be swift and concise in terms of time and distance, but real-world scenarios often present complexities.
Alt text: EPA Federal Test Procedure (FTP) graph showing time and speed variations for emissions testing, alongside an IM240 Inspection & Maintenance Driving Schedule graph, illustrating different drive cycle test durations.
Figure 1 illustrates two examples of emissions testing procedures. The top chart represents the EPA Federal Test Procedure (FTP) process, a comprehensive test lasting 1874 seconds, or over 31 minutes. This rigorous test aims to guarantee a vehicle’s compliance with emissions standards across a wide spectrum of driving conditions. As the graph demonstrates, replicating this exact test in a repair shop setting would be exceedingly challenging and time-consuming.
The lower chart in Figure 1 showcases an example of an IM240 Inspection & Maintenance Driving Schedule. This drive cycle was initially conceived for state emissions programs as a more practical alternative to the lengthy 31-minute FTP test. The IM240 test significantly reduces the duration to just 4 minutes, with speed variations that are more manageable to maintain. However, even this shorter drive cycle can be difficult to execute perfectly on public roads, often necessitating the use of dynamometers in well-equipped shops.
Figure 2 provides a glimpse of a typical OBD II I&M readiness test screen capture. After DTC clearing, battery replacement, or PCM reprogramming, the readiness monitors are initially set to “Incomplete.” As the vehicle undergoes a road test, the PCM executes various system checks and updates the monitor status to “Complete” upon successful completion of these tests.
Alt text: OBD II I&M readiness test screen capture displaying ‘Incomplete’ and ‘Complete’ statuses for various monitors after a drive cycle, indicating system test results.
Important Note: It is crucial to understand that “completed monitors” do not automatically equate to a fully repaired vehicle. Completed monitors simply signify that the PCM is satisfied with the preliminary tests. In numerous instances, more in-depth testing is necessary for a DTC to be triggered.
At this juncture, some technicians in areas without stringent OBD II emissions testing might question the relevance of drive cycles to their practice. The perspective might be that completing I&M readiness monitors is not a primary concern, and customer feedback will suffice to indicate a recurring MIL. However, this approach raises a critical question: How can a customer discern whether a returning MIL is due to the original issue or a new, unrelated problem?
If the MIL illuminates again with the same trouble code, it suggests either a misdiagnosis or the presence of multiple underlying issues, where only one was addressed. Consider the common DTC P0171 (Bank 1 Lean Air Fuel Mixture). While a vacuum leak might have been rectified, a cracked intake air boot could have been overlooked. It is far more advantageous for the technician to identify such issues proactively rather than relying on customer feedback. Furthermore, what if a different system malfunctions after the initial repair? Explaining this to the customer can be challenging, potentially leading to difficulties in billing for additional diagnostic time or even financial losses on the repair. Performing a proper OBDII drive cycle significantly mitigates these risks and enhances customer confidence in the repair work.
Road Test Strategies: From Quick Checks to Comprehensive Cycles
Let’s examine the three road test scenarios outlined earlier. Many technicians rely on a quick road test to validate a repair. A brief road test can be acceptable if there’s a robust method to ensure the initially addressed issue was the sole problem. For example, Mode $06 diagnostics can, in certain situations, verify a repair within a single road test.
As detailed in the article “Advanced Mode $06 Diagnostics,” Mode $06 data can be employed to verify a catalyst repair in a remarkably short timeframe—just over 80 seconds. That article analyzed three distinct repair scenarios, where the first two shops encountered recurring MILs for the same DTC, albeit not immediately after the initial road test. Conversely, the third shop accurately diagnosed the vehicle, replaced the catalytic converter, and utilized Mode $06 to confirm the repair’s effectiveness. Figure 3, originally on page 40 of the source article, illustrates Mode $06$ data updating in real-time during a road test. This exemplifies a DTC-specific drive cycle, which will be explored in greater depth later.
Alt text: Scan tool display showing Mode $06$ data parameters updating dynamically during a vehicle road test, used for real-time repair verification.
The second road-test option involves completing a generic OBD II drive cycle post-repair. A key advantage of this approach is leveraging the PCM to ascertain the proper functioning of all emission-related systems. However, a potential drawback is the extended time required, or the inability to complete the drive cycle due to unfavorable weather conditions or traffic.
Consider a 2000 Lexus LS 400 presenting with a DTC P0133 (Oxygen Sensor Circuit Slow Response). After completing the necessary repair and clearing the code, the customer is obligated to pass the state OBD II emissions program for vehicle registration. In most programs, this mandates that all but one or two readiness monitors must be in a “complete” status.
What are the available options in this scenario? The first is a quick road test, followed by informing the customer that they will need to drive the vehicle for an extended period, hoping the readiness monitors will eventually set. The second, more proactive approach is to perform the OBD II drive cycle on behalf of the customer. This not only validates the repair but also ensures the readiness monitors are completed. But how much time will this consume?
According to Motor’s OBD II Drive Cycle Guide, the process for this particular Lexus (with the MIL off) is as follows:
Step 1: Connect a scan tool to check monitor status and precondition parameters.
Step 2: Start the engine and allow it to idle for a minimum of 2 minutes.
Step 3: Drive at a speed of 25 mph or greater for at least 50 seconds, ensuring engine speed remains above 900 rpm.
Step 4: Stop the vehicle and let the engine idle for at least 40 seconds.
Step 5: Repeat Steps 2 through 4 a total of ten times.
Step 6: Recheck the monitor status; it should now indicate “Complete.” If not, verify that all enabling criteria are met.
Step 7: If the monitor remains “Incomplete,” turn off the ignition and repeat Steps 2 through 5.
Step 8: The readiness status may not transition to “Complete” if a pending DTC is present. Perform a second drive cycle to confirm the DTC. After the second drive cycle, a current DTC will be stored if the issue persists.
For technicians working under a flat-rate compensation model with limited diagnostic time, this process can be inefficient. It’s important to remember this is just one of several monitors requiring completion, potentially extending the overall testing duration significantly.
The crucial takeaway is that researching the appropriate drive cycle should be an integral part of repair planning. A comprehensive understanding of the enabling conditions is essential before initiating the drive cycle. For instance, attempting a drive cycle with an almost empty or completely full fuel tank is counterproductive, as most monitors require a fuel level between 15% and 85%.
If it’s determined that completing the readiness monitors is necessary post-repair, and this will necessitate 30 to 40 minutes of road test time, it’s prudent to recommend additional labor time to accommodate this. Failing to account for this time can lead to financial losses on the repair.
Uncovering Hidden DTCs with OBDII Drive Cycles
Another compelling reason to perform a complete OBD II drive cycle is to detect hidden DTCs. A hidden DTC refers to a fault within the engine management system that remains undetected or unreported due to a higher-priority DTC masking its presence. Figure 4, derived from the 2005 Ford OBD System Operation Summary for Gasoline Engines, illustrates this concept. (These valuable manuals are accessible free of charge at www.motorcraft.com.).
Alt text: Excerpt from a Ford OBD System Operation Summary showing DTC priority logic, indicating how certain sensor faults can prevent other tests, like O2 sensor slow response, from running.
P0133 is a common trouble code that might surface after resolving a different, higher priority DTC. Referring to the “Sensors OK” line in Figure 4, the PCM might not initiate the P0133 O2 Sensor Slow Response Bank 1 test if it has already stored a fault related to the engine coolant temperature (ECT) sensor, intake air temperature (IAT) sensor, or if a misfire DTC P03xx is present.
Consider this scenario: A technician has just completed replacing an ignition coil, resolving a P0305 (Cylinder 5 Misfire) code. After clearing the codes, a road test is performed. Now, with the misfire resolved, the PCM can proceed with the P0133 test. It detects that the Bank 1 Sensor 1 is responding sluggishly and sets a pending DTC. Crucially, most emissions-related DTCs require at least two drive cycle failures to transition from a pending to an active, MIL-illuminating DTC.
The “Monitor Execution” row in Figure 4 indicates that the PCM executes the P0133 test once per drive cycle. This pending DTC might be discovered if the technician reconnects the scan tool and checks for pending codes. However, if this step is skipped, the vehicle might be returned to the customer, and within a day or two, the MIL will likely reappear, leading the customer to believe the initial repair was inadequate.
Understanding DTC enabling conditions is paramount for executing a proper drive cycle following a repair. Figure 4 also presents typical HO2S response rate entry conditions for a P0133 code. For this specific test to run, the Short Term Fuel Trim Range must be within 70% to 130%, and vehicle speed must be maintained between 30 and 60 mph. This explicitly means the test cannot be performed while idling in the service bay. Consequently, a properly functioning vehicle speed sensor (VSS) is essential.
Road-testing the vehicle under these specified conditions significantly increases the likelihood of the PCM accurately testing the Bank 1 O2 sensor and validating the repair. This exemplifies what is meant by road-testing using a DTC-specific drive cycle. Executing this particular drive cycle, once the engine is at operating temperature and in closed-loop operation, can take less than a minute.
In this example, how many drive cycles are needed for the PCM to turn off the MIL? As mentioned earlier, most emissions-related DTCs require at least two drive cycle failures to set. Therefore, it logically follows that at least two successful passes are needed to extinguish the MIL. It’s important to note that the second drive cycle is not considered complete until the ignition is turned off.
Balancing Efficiency and Thoroughness in Drive Cycle Testing
Why might a technician opt for this DTC-specific approach? If the vehicle is in an OBD II emissions testing area and the customer requires immediate state-mandated testing, clearing trouble codes (which resets readiness monitors) might be undesirable. Furthermore, extreme temperatures, such as summer heat in Arizona or winter cold in Alaska, might hinder the effective execution of a complete generic OBD II drive cycle.
The overarching goal of drive cycle testing is to verify the repair. In certain situations, allowing the PCM to turn off the MIL through a few shorter, targeted road tests might be more efficient. However, this is not universally applicable across all vehicles, necessitating thorough research into available options. If time and budget are unlimited, the comprehensive OBD II drive cycle is the ideal choice. However, in most real-world scenarios, time constraints are a significant factor.
Unfortunately, a single, universal drive cycle for all vehicles does not exist. Motor’s OBD II Drive Cycle Guide, when initially published, was nearly 4 inches thick, reflecting the vast complexity. While the information has been updated and is now digitally accessible, the sheer volume of drive cycles remains substantial, likely exceeding a thousand.
Rick Escalambre, a leading instructor at Skyline Community College, has conducted extensive drive cycle testing and developed several standardized drive cycles that prove effective for specific vehicle lines. Figure 5 presents a drive cycle that has shown excellent results for most Chrysler vehicles. More details are available on Escalambre’s website, www.rlescalambre.com.
Alt text: Example of a Chrysler-specific OBDII drive cycle procedure developed by Rick Escalambre, detailing steps for completing readiness monitors efficiently.
Are there any shortcuts or tools that can automatically reset readiness monitors with a single click? Currently, no such tools are known to exist, and their desirability is questionable. Forcibly setting readiness monitors to “complete” would circumvent the crucial PCM verification of the repair’s integrity.
Some vehicle manufacturers offer assistance in guiding vehicles into the correct testing modes. Volkswagen, for instance, provides a “basic settings” mode that guides technicians through a series of in-bay procedures to complete the necessary drive cycle tests. However, this process still requires time and is specifically designed to ensure the thorough completion of all readiness monitors.
Chrysler offers an exemplary format for readiness monitor preparation. Figure 6 showcases screenshots from the DRB III factory scan tool, illustrating the testing of the exhaust gas recirculation (EGR) system monitor. The required parameters are displayed with both low and high range limits, while the center column presents the current data readings. In the top screen, the first four parameters are within range, but the “RPM Range” is highlighted, indicating the RPM is too low. The technician must increase the RPM to meet the PCM’s criteria.
Alt text: Chrysler DRB III scan tool interface displaying EGR system monitor test parameters, showing required ranges and current values, guiding technicians to meet enabling conditions.
The lower screen in Figure 6 demonstrates that all enabling criteria have been satisfied, and the test is in progress. Once the test is complete, the scan tool screen will display “Done This Trip.” Adhering to this type of guided procedure ensures both proper vehicle repair and thorough testing.
Figure 7 illustrates Toyota’s approach to displaying EGR monitor data. Three key data points are presented from the scan data: EGRT Gas, EGR System, and EGR Monitor. The leftmost screen shows the EGR temperature (EGRT) at 66.2°F, EGR commanded “OFF,” and the monitor status as “Incomplete.” The middle screen shows the EGR commanded “ON” and the EGRT beginning to rise. The rightmost screen shows the continued increase in EGRT, with the EGR monitor now displaying “Complete.”
Alt text: Toyota scan tool interface showing EGR monitor data progression across three screens, illustrating EGR temperature increase and monitor status change from ‘Incomplete’ to ‘Complete’ as test conditions are met.
The scan tool reigns supreme when it comes to understanding the PCM’s diagnostic process. Investing in a scan tool with robust capabilities is essential. Relying solely on a generic OBD II scan tool will present significant limitations in effectively diagnosing and verifying complex repairs.
The Horizon of OBDII: Permanent DTCs and Future Implications
Looking ahead, it’s important to be aware of Mode $0A$ Permanent DTCs, a significant update to the OBD II specification. While most technicians may not encounter this technology immediately, it is on the horizon. Permanent DTCs possess a unique characteristic: they can only be erased by the PCM itself, rendering scan tools or battery disconnects ineffective for clearing them.
State emissions programs are poised to begin leveraging this data to identify instances where technicians might be clearing DTCs and performing minimal road tests solely to achieve readiness monitor completion, temporarily satisfying state requirements. This aims to target vehicles that are not genuinely repaired, where the MIL may reappear shortly after passing the emissions test. While currently informational, permanent DTCs may become integral to future regulations.
Ultimately, the paramount objective is to repair vehicles correctly the first time. Drive cycle testing is a valuable tool designed to aid in repair validation, but its effectiveness hinges on proper execution. By understanding and correctly applying OBDII drive cycle procedures, auto repair professionals can ensure repair quality, minimize comebacks, and enhance customer trust.
