For automotive technicians, navigating the complexities of driveability diagnostics can often feel like searching for a needle in a haystack. A systematic approach is crucial, and one of the most effective starting points is leveraging the power of a scan tool. The question then arises: “Which scan tool is the right choice?” While a factory scan tool might be ideal in a perfect world with unlimited resources, generic OBD II scan tools offer a practical and highly effective solution for the majority of diagnostic scenarios.
In fact, approximately 80% of driveability issues encountered can be effectively diagnosed or significantly narrowed down using nothing more than the generic parameters available through an OBD II scan tool. Furthermore, these tools are accessible and affordable, with capable options available for under $300.
The landscape of OBD II diagnostics has become even more powerful with the evolution of standardized parameters. Early OBD II systems offered a valuable set of data, but recent advancements, particularly with CAN-equipped vehicles, have dramatically expanded the scope and depth of information available. Figure 1, derived from a 2002 Nissan Maxima (as shown in the original article), exemplifies the typical parameters of older OBD II vehicles, often ranging from 13 to 20 parameters out of a possible 36 under the initial specification. However, revisions driven by the California Air Resources Board (CARB) for OBD II CAN-equipped vehicles have ushered in an era of richer data, potentially exceeding 100 generic parameters. Figure 2, showcasing data from a CAN-equipped 2005 Dodge Durango (as shown in the original article), vividly illustrates this leap in both the quantity and quality of diagnostic data.
This article will delve into the essential OBD II parameters that provide the most insightful diagnostic information, with a particular focus on the newly phased-in parameters, including Obdii Ol Drive, and how to effectively utilize them for superior automotive diagnostics.
Decoding Fuel Trim: Your First Step in Driveability Diagnostics
Regardless of the specific driveability symptom, Short-Term Fuel Trim (STFT) and Long-Term Fuel Trim (LTFT) should always be your initial parameters of focus. Fuel trim acts as a critical diagnostic window into the engine control module (PCM)’s fuel delivery adjustments and the adaptive fuel strategy in operation. Expressed as percentages, STFT and LTFT ideally operate within a ±5% range.
- Positive fuel trim percentages indicate the PCM is adding fuel (enriching the mixture) to compensate for a perceived lean condition.
- Negative fuel trim percentages signal the PCM is reducing fuel (leaning the mixture) to counteract a perceived rich condition.
STFT typically fluctuates rapidly as the PCM makes real-time adjustments, while LTFT represents more stable, learned corrections over time. If either STFT or LTFT consistently exceeds ±10%, it serves as a strong indicator of a potential underlying issue requiring further investigation.
To effectively utilize fuel trim for diagnosis, it’s crucial to assess the readings across different engine operating ranges. Check fuel trim at idle, 1500 RPM, and 2500 RPM. For instance, if LTFT Bank 1 (B1) shows +25% at idle but corrects to within ±5% at 1500 and 2500 RPM, the diagnostic focus should narrow to factors causing a lean condition specifically at idle, such as a vacuum leak. Conversely, if a fuel trim issue persists across all RPM ranges, the likely cause points towards fuel supply problems, such as a failing fuel pump or restricted fuel injectors.
Furthermore, fuel trim can help isolate issues to specific cylinder banks in bank-to-bank fuel control engines. For example, if LTFT B1 reads -20% while LTFT B2 is at +3%, the problem originates within Bank 1 cylinders, guiding your diagnostic efforts to that specific engine bank.
Expanding Your Diagnostic View: Key OBDII Parameters Beyond Fuel Trim
Beyond fuel trim, numerous other OBDII parameters offer valuable diagnostic insights, even when fuel trim readings appear normal. These parameters can reveal underlying issues or provide corroborating evidence to support your diagnosis:
- Fuel System 1 Status & Fuel System 2 Status: These parameters should ideally display “Closed Loop” (CL) operation. If the system is in “Open Loop” (OL), fuel trim data might be unreliable. Crucially, newer OBDII standards provide more detailed fuel system status information, including OL-Drive, OL-Fault, and CL-Fault, which we will explore in detail later.
- Engine Coolant Temperature (ECT): The ECT should reach and maintain normal operating temperature, ideally 190°F (88°C) or higher. A consistently low ECT reading can cause the PCM to incorrectly richen the fuel mixture, skewing fuel trim data.
- Intake Air Temperature (IAT): IAT should reflect ambient temperature or the temperature under the hood, depending on sensor location. When the engine is cold (Key On Engine Off – KOEO), ECT and IAT readings should be within approximately 5°F (3°C) of each other.
- Mass Airflow (MAF) Sensor: If equipped, the MAF sensor measures incoming air mass, a critical input for PCM fuel calculations. Verify MAF sensor accuracy across various RPM ranges, including Wide Open Throttle (WOT), and compare readings to manufacturer specifications. Pay close attention to the units of measurement (grams per second – gm/S or pounds per minute – lb/min) to avoid misinterpretations.
- Manifold Absolute Pressure (MAP) Sensor: If present, the MAP sensor measures manifold pressure, indicating engine load. Readings are typically displayed in inches of mercury (in./Hg). Remember that MAP sensor readings are not direct vacuum readings; vacuum can be calculated using the formula: Barometric Pressure (BARO) – MAP = Intake Manifold Vacuum. Some vehicles utilize MAF sensors, some MAP sensors, and some employ both.
- Oxygen Sensor Output Voltage (B1S1, B2S1, B1S2, etc.): Oxygen sensors are vital for fuel mixture control and catalytic converter efficiency monitoring. Use your scan tool to assess basic sensor operation. Sensors should rapidly fluctuate between below 0.2 volts and above 0.8 volts during normal operation. A “snap throttle” test can often verify this voltage swing. For more in-depth oxygen sensor analysis, a graphing scan tool is highly beneficial.
- Engine Speed (RPM) & Ignition Timing Advance: These parameters are crucial for evaluating idle control strategy and are best analyzed using a graphing scan tool to observe their dynamic behavior.
- Vehicle Speed Sensor (VSS) & Throttle Position Sensor (TPS): Verify the accuracy of RPM, VSS, and TPS readings. These parameters serve as valuable reference points for symptom duplication and problem isolation during data recording.
- Calculated Load, MIL Status, Fuel Pressure, & Auxiliary Input Status (PTO): Monitor these parameters if available, as they can provide supplementary diagnostic information.
Unlocking Advanced Diagnostics with New OBDII Parameters: Understanding OL-Drive and Beyond
The expansion of OBDII parameters in newer vehicles significantly enhances diagnostic capabilities. Introduced on 2004 CAN-equipped vehicles and potentially present on earlier or non-CAN models, these new parameters offer a deeper understanding of engine operation. Let’s examine some key examples, focusing on OBDII OL Drive and its related fuel system statuses:
- FUEL STAT 1 = Fuel System 1 Status (and FUEL STAT 2): This parameter provides a more granular view of fuel system status than the basic “Open Loop” (OL) or “Closed Loop” (CL) indicators. Crucially, it introduces statuses like:
- OL-Drive: Open Loop – Drive. This status signifies an open-loop condition specifically during power enrichment (increased fuel delivery for high power demand, such as acceleration) or deceleration enleanment (reduced fuel delivery during deceleration to improve fuel economy and emissions). OL-Drive is a normal operating mode under these specific driving conditions. Seeing OL-Drive during acceleration or deceleration is typically not indicative of a fault. However, if OL-Drive persists during steady-state cruising or idle, it could suggest an issue preventing closed-loop operation under normal conditions.
- OL-Fault: Open Loop – Fault. This indicates the PCM is commanding open-loop operation due to a detected system fault. This is an abnormal condition and signals a problem preventing the system from entering closed-loop control. Further investigation is required to identify the fault triggering OL-Fault.
- CL-Fault: Closed Loop – Fault. This status suggests the PCM is operating in closed-loop but employing an alternative fuel control strategy due to an oxygen sensor fault or other issues. While in closed-loop, the system isn’t functioning optimally due to a fault.
Understanding these nuanced fuel system statuses, particularly OBDII OL Drive, is critical for accurate diagnostics. Seeing OL-Drive under expected conditions is normal; however, its presence outside of power enrichment or deceleration scenarios warrants further investigation.
- ENG RUN TIME = Time Since Engine Start: Useful for pinpointing when intermittent problems occur within an engine run cycle.
- DIST MIL ON = Distance Traveled While MIL Is Activated: Provides insight into how long a problem has persisted, potentially indicating the severity or customer neglect.
- COMMAND EGR = EGR_PCT: Displays commanded Exhaust Gas Recirculation (EGR) as a percentage. Useful for verifying EGR system operation, but remember it only reflects commanded EGR, not actual flow.
- EGR ERROR = EGR_ERR: Indicates EGR position errors as a percentage, calculated based on the difference between actual and commanded EGR position. High EGR Error readings can point to EGR valve or sensor malfunctions.
- EVAP PURGE = EVAP_PCT: Shows commanded Evaporative Emission (EVAP) purge as a percentage. Crucial for diagnosing fuel trim anomalies, as normal EVAP purge operation can influence fuel trim readings. Temporarily blocking the purge valve can help isolate EVAP purge as a factor in fuel trim issues.
- FUEL LEVEL = FUEL_PCT: Essential for system monitor completion and diagnosing fuel level-dependent issues, particularly for misfire and evaporative emissions monitors that often have fuel level thresholds for activation.
- WARM-UPS = WARM_UPS: Counts warm-up cycles since DTCs were cleared, aiding in duplicating codes requiring specific warm-up cycles for completion. A warm-up is defined as a significant temperature rise and reaching a minimum temperature.
- BARO = BARO: Barometric pressure reading, valuable for diagnosing MAP and MAF sensor accuracy, especially considering altitude variations.
- CAT TMP B1S1/B2S1 = CATEMP11, 21, etc.: Catalyst temperature readings, crucial for assessing catalytic converter operation and diagnosing premature catalyst failure due to overheating.
- CTRL MOD (V) = VPWR: PCM voltage supply, often overlooked but critical. Low voltage can cause various driveability problems. This parameter reflects voltage at the PCM, but remember other voltage supplies (like ignition voltage) are also critical and may require enhanced scan tools or direct measurement to check.
- ABSOLUT LOAD = LOAD_ABS: Normalized air mass per intake stroke, indicating engine load as a percentage. Used by the PCM for spark and EGR scheduling and for diagnostic assessment of engine pumping efficiency.
- OL EQ RATIO = EQ_RAT: Commanded equivalence ratio, used to determine the commanded air/fuel ratio. Displays 1.0 in closed-loop for conventional oxygen sensor vehicles and the PCM-commanded ratio in open-loop. Wide-range oxygen sensors display the commanded ratio in both open and closed loop.
- TP-B ABS, APP-D, APP-E, COMMAND TAC: Throttle-by-wire system parameters, essential for diagnosing electronic throttle control issues.
Furthermore, enhanced OBDII data often includes misfire counts for individual cylinders and wide-range air/fuel sensor readings in voltage or milliamp units.
Scan tools may also provide valuable information about data communication on the CAN bus. Symbols like “>” (multiple ECUs responding with different values), “=” (multiple ECUs supporting and providing similar values), and “!” (no response received for a supported parameter) can aid in diagnosing CAN bus communication problems.
Maximizing Your OBDII Scan Tool Effectiveness
OBDII generic data has evolved into a powerful diagnostic resource. The key to effective utilization lies in understanding each parameter, its interrelationships, and its diagnostic significance, especially parameters like OBDII OL Drive.
When selecting an OBDII scan tool, prioritize models with graphing and recording capabilities. These features significantly enhance diagnostic efficiency and data analysis. While generic OBDII specifications provide a valuable standardized framework, always remember to consult vehicle-specific service information for potential variations and manufacturer-specific specifications.
By investing in a capable OBDII scan tool and mastering the interpretation of generic parameters, including the advanced statuses like OL-Drive, automotive technicians can elevate their diagnostic skills and confidently tackle even complex driveability challenges.
