For automotive technicians tackling driveability issues, a systematic approach is crucial. Often, the diagnostic journey begins with a scan tool, prompting the common question: “Which scan tool is best?”. While a factory scan tool would be ideal in a limitless world, budget constraints often lead professionals and enthusiasts to more accessible options. A generic OBD II scan tool emerges as a highly effective starting point. Remarkably, around 80% of driveability problems can be effectively diagnosed or significantly narrowed down using the generic parameters available through these tools, often priced under $300.
The landscape of OBD II diagnostics is continuously evolving, with recent parameter expansions significantly enhancing the value of generic data. Consider Figure 1, illustrating the parameter set from a 2002 Nissan Maxima. Original OBD II specifications offered up to 36 parameters, with vehicles of that era typically supporting 13 to 20. However, revisions driven by the California Air Resources Board (CARB) for OBD II CAN-equipped vehicles have broadened the scope to over 100 potential generic parameters. Figure 2 showcases data from a 2005 Dodge Durango, demonstrating the substantial increase in both the quality and quantity of accessible data. This article will guide you through the most informative parameters, including the newly introduced ones, with a specific focus on Command Tac Obd2 and its role in diagnostics.
Irrespective of the specific driveability symptom, Short-Term Fuel Trim (STFT) and Long-Term Fuel Trim (LTFT) should be your initial focus. Fuel trim acts as a vital diagnostic window into the engine control module (PCM)’s fuel delivery adjustments and adaptive strategies. Expressed as percentages, ideal fuel trim values reside within ±5%. Positive percentages indicate the PCM is enriching the mixture to compensate for a lean condition, while negative values signal enleanment to counter a rich condition. STFT typically fluctuates rapidly, whereas LTFT remains more stable. Values exceeding ±10% in either STFT or LTFT warrant further investigation.
To deepen your analysis, assess fuel trim across different engine speeds: idle, 1500 rpm, and 2500 rpm. For instance, a LTFT B1 of 25% at idle that corrects to 4% at higher RPMs suggests a vacuum leak, causing a lean condition specifically at idle. Conversely, if the issue persists across all RPM ranges, suspect fuel supply problems like a failing fuel pump or clogged injectors.
Fuel trim can also pinpoint problems to specific cylinder banks in bank-to-bank fuel control systems. A reading of LTFT B1 at -20% and LTFT B2 at 3% implicates a problem confined to bank 1 cylinders, directing your diagnostic efforts accordingly.
Beyond fuel trim, several other OBD II parameters offer valuable diagnostic insights:
Fuel System 1 Status and Fuel System 2 Status should ideally be in “Closed Loop” (CL). Inability to achieve CL can compromise the accuracy of fuel trim data.
Engine Coolant Temperature (ECT) needs to reach and maintain operating temperature, ideally 190°F or higher. Low ECT readings might trigger the PCM to erroneously enrich the fuel mixture, mimicking a cold start condition.
Intake Air Temperature (IAT) should reflect ambient or underhood temperature, depending on sensor location. During a cold engine check (Key On Engine Off – KOEO), ECT and IAT should be within 5°F of each other.
The Mass Airflow (MAF) Sensor (if equipped) measures incoming air mass, crucial for the PCM’s air-fuel mixture calculations. Verify MAF sensor accuracy across RPM ranges, including Wide Open Throttle (WOT), comparing readings against manufacturer specifications. Remember to check the units – grams per second (gm/S) or pounds per minute (lb/min) – to avoid misinterpretations.
The Manifold Absolute Pressure (MAP) Sensor, if present, measures manifold pressure, indicating engine load to the PCM. Readings are typically in inches of mercury (in./Hg). Avoid confusing MAP with intake manifold vacuum; they are related but distinct. Intake manifold vacuum can be calculated as: Barometric Pressure (BARO) – MAP = Intake Manifold Vacuum. Vehicles may utilize MAF, MAP, or both.
Oxygen Sensor Output Voltage (B1S1, B2S1, B1S2, etc.) is used for fuel mixture control and catalytic converter efficiency monitoring. Scan tools facilitate basic sensor operation checks. Sensors should fluctuate rapidly, exceeding 0.8 volts and dropping below 0.2 volts during transitions. A snap throttle test can often verify these voltage limits. For slower transitions, lab scope testing is recommended before sensor replacement. Remember that OBD II generic data reporting is not real-time; data is processed by the PCM before being relayed to the scan tool, with data rates typically around 10 samples per second for a single parameter, and slower when monitoring multiple parameters. Graphing individual oxygen sensors can optimize data interpretation.
Engine Speed (RPM) and Ignition Timing Advance are valuable for assessing idle control, best analyzed using a graphing scan tool.
RPM, Vehicle Speed Sensor (VSS), and Throttle Position Sensor (TPS) accuracy is crucial. These parameters also serve as valuable reference points for symptom duplication and data logging for intermittent issues.
Calculated Load, MIL Status, Fuel Pressure, and Auxiliary Input Status (PTO) can provide supplementary diagnostic information when available.
Exploring Enhanced OBD II Parameters, Including Command TAC OBD2
The advent of CAN-equipped vehicles in 2004 brought a wealth of new OBD II parameters, although some may appear on earlier or non-CAN models. Figure 2, data from a 2005 Dodge Durango, exemplifies these expanded parameters. Here’s a breakdown of some key additions:
FUEL STAT 1 (Fuel System 1 Status): Provides more detailed fuel system status beyond simple “Open Loop” (OL) or “Closed Loop” (CL). Possible statuses include “OL-Drive” (open loop during power enrichment or deceleration enleanment), “OL-Fault” (open loop due to system fault), and “CL-Fault” (potentially altered fuel control strategy due to oxygen sensor fault).
ENG RUN TIME (Time Since Engine Start): Useful for pinpointing when specific problems occur during an engine cycle.
DIST MIL ON (Distance Traveled While MIL Is Activated): Indicates how long a problem has persisted, valuable for customer communication and understanding the severity of a recurring issue.
COMMAND EGR (EGR_PCT): Displays commanded Exhaust Gas Recirculation (EGR) as a normalized percentage across EGR systems. 0% indicates EGR OFF/Closed, and 100% represents fully open. Crucially, this reflects commanded position, not actual flow rate.
EGR ERROR (EGR_ERR): Shows EGR position errors as a percentage, also normalized. Calculated as: (Actual EGR Position – Commanded EGR) / Commanded EGR. High EGR Error readings when EGR is commanded OFF can indicate a stuck EGR valve or sensor malfunction.
EVAP PURGE (EVAP_PCT): Displays Evaporative Emission Control System purge command as a normalized percentage. Essential for diagnosing fuel trim anomalies, as normal purge operation can influence fuel trim. To isolate EVAP purge as a factor, temporarily block the purge valve inlet and re-evaluate fuel trim.
FUEL LEVEL (FUEL_PCT): Critical for system monitor completion and specific diagnostics. Many monitors, like misfire monitors on certain models, have fuel level thresholds for activation (e.g., >15% for a 1999 Ford F-150 misfire monitor). Evaporative emission monitors often have fuel level ranges (e.g., 15%-85%).
WARM-UPS (WARM_UPS): Counts warm-up cycles since DTCs were cleared. A warm-up is defined by a 40°F ECT rise from start temperature, reaching a minimum of 160°F. Useful for verifying warm-up cycle completion requirements for specific codes.
BARO (BARO): Barometric pressure reading, essential for diagnosing MAP and MAF sensor issues. Verify KOEO accuracy relative to your altitude.
CAT TMP B1S1/B2S1 (CATEMP11, 21, etc.): Catalyst temperature, either directly measured or inferred. Valuable for assessing catalyst operation and diagnosing premature failures due to overheating.
CTRL MOD (V) (VPWR): PCM voltage supply. Surprisingly absent in the original OBD II spec, this parameter is critical. Readings should closely match battery voltage. Low voltage can cause driveability problems. Note that ignition voltage supply, another potential issue, usually requires enhanced scan tools or direct measurement.
ABSOLUT LOAD (LOAD_ABS): Normalized air mass per intake stroke, displayed as a percentage. Ranges from 0%-95% for naturally aspirated engines and 0%-400% for boosted engines. Used for spark and EGR scheduling and engine pumping efficiency diagnostics.
OL EQ RATIO (EQ_RAT): Commanded equivalence ratio, indicating the commanded air-fuel ratio. Closed-loop readings should be 1.0 for conventional oxygen sensor vehicles. Wide-range/linear oxygen sensors display commanded EQ ratio in both open and closed loop. Actual Air-Fuel Ratio = Stoichiometric A/F Ratio x EQ Ratio (e.g., for gasoline, 14.64:1 stoichiometric ratio x 0.95 EQ ratio = 13.9:1 A/F).
TP-B ABS, APP-D, APP-E, COMMAND TAC: These parameters, specifically COMMAND TAC, relate to throttle-by-wire systems, as seen on the 2005 Dodge Durango. Command TAC OBD2 is crucial for diagnosing electronic throttle control issues. COMMAND TAC represents the Throttle Actuator Control command, indicating the PCM’s desired throttle plate position. Deviations between COMMAND TAC and actual Throttle Position Sensor readings can pinpoint problems within the throttle-by-wire system itself, or in related circuits. Other throttle-by-wire parameters may vary across vehicle makes and models.
Further enhanced parameters, though not always universally available, include misfire data per cylinder (similar to GM enhanced scan tools) and wide-range/linear air-fuel sensor readings in voltage or milliamp units.
Figure 5 illustrates ECU response indicators from a Vetronix MTS 3100 Mastertech. The “>” symbol signifies differing values for a parameter across multiple ECUs. The “=” symbol indicates parameter support by multiple ECUs with similar values. “!” denotes no response for a parameter that should be supported, aiding in CAN bus data issue diagnosis.
In conclusion, OBD II generic data, including parameters like Command TAC OBD2, has evolved significantly and offers substantial diagnostic value. Thoroughly examining each parameter and understanding their interrelationships is key. If you haven’t yet invested in a generic OBD II scan tool, prioritize models with graphing and recording capabilities. These features will quickly prove their worth. While mastering new parameters requires time, their diagnostic potential is undeniable. Always remember that OBD II generic specifications can have vehicle-specific variations; consulting vehicle service information for specific details and specifications remains essential for accurate diagnosis.
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