Computerized Powertrain Controls Diagnosis (Including OBD II)

Electronic engine control systems have been standard equipment on most cars and light trucks since the early 1980s. While these systems improve both driveability and fuel economy, their primary purpose is emission control. From a service standpoint, many of the early systems were difficult to diagnose due to their limited self-diagnostic capability. However, serviceability improved in 1988 with the implementation of OBD I legislation. This ruling required all vehicles to have a dash mounted warning light that would illuminate if a failure occurred in the fuel metering system, sensor network, EGR system, and/or computer. While the OBD I ruling improved the functionality of engine control systems, its standards were too lax to make a significant difference in overall air quality or serviceability. As a result, the California Air Resources Board developed new standards for on-board diagnostics in 1989, which eventually resulted in OBD II.


Communicating with a vehicle’s on-board diagnostic system begins at the DLC (Data Link Connector). On OBD I systems, the shape and location of the connector is unique to each vehicle manufacturer. With OBD II, the DLC has been standardized into a 16-pin design complete with power and ground circuits at pins 4 (ground) and 16 (B+) . These circuits eliminate the need for a separate power supply cable when connecting a scan tool. According to OBD II regulations, seven cavities of the DLC are common to all vehicles (as defined by the Society of Auto¬≠ motive Engineers), while the remaining nine are proprietary (manufacturer specific). Unlike the connectors used for OBD I systems, OBD II vehicles are required to have the DLC located close to the instrument panel within the area that extends from the driver’s door to 12 inches (300mm) beyond the vehicle centerline. On most vehicles, you should be able to spot the DLC from a crouched position with the driver’s door open.


Depending on the vehicle, the MIL may be displayed as ‘Check Engine,’ ‘Service Engine Soon,’ or the ISO engine symbol. Unlike OBD I systems, the MIL on an OBD II vehicle is not used for ‘flash diagnostics.’ Consequently, a scan tool must be used to retrieve OBD II trouble codes. On certain OBD II vehicles however, the MIL may be used to ‘flash’ manufacturer specific, two digit codes. In terms of failure detection, OBD I systems operate the MIL based on current failures. In other words, the MIL will only be illuminated at the time a monitored circuit is malfunctioning. If the problem disappears, the MIL will go out. In contrast, the MIL will remain on in an OBD II system until the vehicle completes three consecutive trips without a repeat failure. This means that the MIL may be on even though there are no current failures. For the most serious emissions related failures, including misfires and fuel trim problems, the MIL will stay illuminated until the vehicle completes three consecutive trouble-free trips under nearly the same conditions that occurred during the initial failure. This ‘similar condition’ strategy prevents false codes or soft codes which were common on OBD I systems.


With OBD I, each manufacturer used their own unique codes and definitions for identifying failures in the engine management system. This became a burden for technicians servicing multiple vehicle brands. With OBD II, common codes and definitions were developed to identify all basic emissions-related failures. OBD II trouble codes consist of one alpha character followed by four digits. The alpha character indicates the area of the vehicle where the failure occurred. This includes (B) Body, (C) Chassis, (P) Powertrain, and (U) Network. The first digit of the DTC denotes the origin of the code. Codes authored by the Society of Automotive Engineers (SAE) are identified by a zero (0). These codes are known as generic DTCs since they are the same for every vehicle. Manufacturer specific codes are indicated by the number one (1). These DTCs are part of the manufacturer’s enhanced diagnostic software, and vary between brands. The second digit in the DTC identifies the system experiencing the problem, while the last two digits correspond to a specific code definition.


OBD I and OBD II are similar in that both systems check sensor and actuator circuits for opens, shorts and out-of-range values. However, the failure limits for OBD I are far more forgiving, since a circuit or component must fail completely before the MIL is illuminated or a DTC is stored. In contrast, OBD II uses a series of monitors (diagnostic tests) that conduct performance evaluations on emission components and subsystems. If a monitored circuit fails to meet minimum performance standards, even though the circuit may still be operational, the ECM (Engine Control Module) or PCM (Powertrain Control Module) will illuminate the MIL and store a DTC. This capability makes it possible for emissions related problems to be identified and corrected before excessive pollutants are discharged into the atmosphere.

OBD II monitors include:

  • Comprehensive Components
  • Misfire Detection
  • Fuel Control
  • Exhaust Gas Recirculation (EGR)
  • Catalyst Efficiency
  • Oxygen Sensor
  • 02 Heater
  • Evaporative Emissions (EVAP)
  • NC Refrigerant
  • Heated Catalyst
  • Positive Crankcase Ventilation (PCV)
  • Secondary Air
  • Thermostat


OBD II monitors are defined as being continuous or non-continuous. As the name indicates, continuous monitors run all the time. These monitors include Comprehensive Components, Misfire Detection and Fuel Control. The remaining monitors are non-continuous, since they do not run until certain ‘enable’ criteria has been met. Enable criteria include specific driving and engine operating conditions that must occur before the ECM will execute the monitor. Consequently, if the vehicle is driven in a way that does not satisfy the enable criteria for a particular non-continuous monitor, that monitor will not run.


Monitor readiness is a required test function for an OBD II scan tool. In this mode, all of the OBD II monitors are displayed along with corresponding messages indicating their execution status (run/not run). Monitor readiness DOES NOT indicate whether a given monitor has passed or failed. Since continuous monitors run all the time, their status will always be displayed on the scan tool as having run (e.g. YES , READY, COMPLETE). Where non-continuous monitors are concerned, they will be displayed as not having run (e.g. NO , NOT READY, INCOMPLETE) unless the vehicle was operated in a way that satisfied the appropriate enable criteria . Non-continuous monitors will also be displayed as not having run following a battery or ECM disconnect, or if DTCs are cleared with the scan tool.


Scan data is a list of Parameter IDs (PIDs) indicating operational values of powertrain components . When diagnosing a system malfunction , scan data should be carefully reviewed under KOEO and KOER (Key On, Engine Run) conditions. OBD II systems provide two forms of scan data: Generic and Enhanced. Generic data, consists of a limited number of PIDs, such as engine coolant temperature and oxygen sensor voltages. All OBD II systems display generic data. Enhanced data, which is manufacturer specific information, is a complete list of parameters covering all of the vehicle’s powertrain inputs and outputs . On scan tools with a limited viewing area, PIDs are typically abbreviated to conserve screen space. For example, ‘CLOSED LOOP’ may be displayed as ‘CLOS LP.’ Parameter values will also be abbreviated if screen space is tight, as in the case of ‘APP’ for APPLIED and ‘REL’ for RELEASED. Once the scan tool has been connected to the vehicle and programmed properly, check to see if there are any stored codes in the ECM. Even if the MIL was off when the vehicle came in, there may be a history code in memory that can help isolate the root cause of the problem. The next step is to examine the scan data to see if sensor and actuator values are within their intended range. In theory, out-of-range sensor and/or actuator values should always be accompanied by a DTC. However, this is not always the case,
especially on OBD I systems. This is why it’s important to compare actual scan data readings to the desired values listed in the service manual. Although scan data is a valuable source of diagnostic information, never replace suspect components based on this information alone. This is especially true when diagnosing no-code complaints. Always confirm the accuracy of the scan readings by performing additional tests using the appropriate equipment.


Using an ohmmeter to evaluate the condition of an electrical circuit is a common mistake made by technicians. To illustrate this, consider using an ohmmeter to check the condition of a battery cable. While a low resistance reading may prove that the cable has continuity, it provides little evidence that the cable could support the high current demands of the starter. This is because an ohmmeter only sends a few milliamps of current through the circuit under test. Consequently, you could get a low ohm reading from a battery cable that only has one strand of wire intact. The problem is, the engine would never crank using a cable in that condition. While an ohmmeter is effective for identifYing open or shorted circuits or checking the resistance of electrical components, it cannot measure the energy losses that occur in a live circuit. Anytime electrical energy is consumed in an area other than its intended purpose, the circuit will malfunction. That’s why checking voltage drop is so important . A voltage drop test measures the difference in electrical pressure between two points in a live circuit. In order to obtain accurate test results, the normal maximum current must be flowing. For example, since most powertrain relays and solenoids are energized when the engine is running, computer grounds should always be checked under KOER conditions. Otherwise, excessive voltage drop may not be apparent. To check the voltage drop across a computer ground , connect the negative lead to the engine block and the positive lead to the appropriate terminal at the computer. With the engine running, the voltage should not exceed 100 millivolts (O.001V). If voltage is higher than this, move the positive probe to the next closest point in the circuit. This may be a connector, splice, or junction block. Look for a reduction in voltage each time the positive probe is moved to a new location . Once the voltage reading drops to within limits, it indicates that high resistance exists between the positive probe’s current location and the last point checked. If the voltage remains high at all points, check for excessive resistance between the alternator and the engine.


Before replacing an ECM to correct a problem in an actuator circuit (e.g. inoperative cooling fan), always perform a current draw test to ensure that the computer is the root cause of the problem. This is especially important on older OBD I systems, since the computer can become permanently damaged if current in a driver circuit exceeds 750 milliamps. This would be the case if a relay or solenoid were shorted. Although driver circuits are fault-protected to prevent computer damage on late-model systems, excessive current will cause the ECM to shutdown the problem circuit. For example, if the cooling fan relay is shorted, excessive current will flow when the ECM completes the ground for the relay coil. On older OBD I systems, the high current will typically burn the driver open, making it necessary to replace the computer. On a late-model vehicle, the circuit will open automatically. In either case however, the symptom would be an inoperative cooling fan. To check current draw, set the multimeter to the amps position, and move the positive lead to the amp jack. Next, unplug the appropriate ECM harness connector, and connect the positive probe to the driver terminal you need to check. Attach the negative probe to a good ground preferably the negative battery terminal. Now, turn the key to the RUN position. At this point, the device should turn on and the meter should indicate the current flow. Under normal conditions, current flow will be less than 750 milliamps. However, if current is greater than this, the appropriate actuator (relay or solenoid) should be replaced. On older vehicles, the computer may also require replacement if circuit current is greater than 750 milliamps.


More and more vehicles are increasingly using another level of electronic control. This protocol is SAE standard J1939, which is referred to as Common Area Network (CAN). CAN takes advantage of one of the most fundamental advantages of an electronic processor, which is the ability to examine data an infinite number of times as desired to add or enhance features. The use of this protocol allows all controllers on the vehicle to share inputs. For example, a single vehicle speed sensor can be used and wired to program the transmission controller, the anti-lock brake controller, and the instrument panel can also use the input from that single sensor. This significantly reduces the amount of wiring and components required on a vehicle. A CAN harness consists of a shielded two-wire twisted pair harness. The use of a twisted pair harness reduces interference that could affect communication on the data link. By twisting the wires, the magnetic field surrounding one wire will cancel the magnetic field surrounding the other wire. This results in an electrical signal that is ‘clean’ of any magnetic interference. The two wires are called CAN High (CAN H) and CAN Low (CAN L). All controllers on the vehicle are wired in a parallel circuit to the CAN harness. CAN H has a dormant voltage of 0.25 volts that rises to 0.65 volts when communicating. Data is seen when this signal is at this ‘high’ voltage level. CAN L has a dormant voltage of 11 volts that drops to 4.65 volts when active, and this ‘low’ voltage is seen as data. CAN presents the technician with a new set of diagnostic challenges. Up to this point in the advancement of vehicle technology one constant factor existed; One wire carried one electrical value. If all else failed, the technician could always use a lab scope to examine even the fastest of electrical signals. While CAN still carries one signal at a time, the data transmission is too fast to be deciphered on a scope pattern. Some manufacturers have developed diagnostic software to assist the technician in determining the root cause of a defect. This software is usually laptop based, and the connection is made at the same diagnostic data link used for OBD II troubleshooting. A terminating resistor is at each end of the data link, terminating resistors serve to absorb signal ‘bounce’ and absorb interference. A terminating resistor may be found on the harness, or it may be integral with a controller. While the data link can operate if one resistor fails, the failure of both resistors will usually shut down the data link. In addition , shorted or open harnesses may cause a loss of communication with an individual controller. The presence and condition of terminating resistors can be easily checked with an ohmmeter at the OBD II diagnostic connector. With all power removed, measure resistance from pin 6 to pin 14 of the diagnostic connector. Normal resistance should be 60 ohms. One open or missing resistor would show a resistance reading of 120 ohms.

This post was written by: Martin Hand


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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.


  • GLENN ZALETA says:

    dear martin i would appreciate your help ,I have a 04 subaru legacy l 35 Anniversary edition and i cant pass NY state inspection because when you hook up to the OBD-2 connection the commputer says no communication to ecm ,I have power ground to scaner ,I have check all ground curcuit with OHM meter ok iam going to check resistance to pin 6 and 14 and see if it ok ,IF YOU COULD HELP ME IT WOULD BE GREAT.

    • Martin Hand Martin Hand says:

      If the scanner powers up and says no communication then power and grounds are okay at the OBDII connector, check your bus line for open or shorted circuit. A failed module on the communication circuit can cause this also.

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