2. Chapter Objectives
Prepare for the ASE Engine Performance (A8) certification test content area “E” (Computerized Engine Controls Diagnosis and Repair).
Explain the purpose and function of OBD II.
Describe how the PCM performs active and passive tests of the computerized engine controls system.
Describe the standardized OBD-II DTCs and terminology.
Discuss continuous and non-continuous monitors.
Explain the difference between enhanced and generic OBD II.

3. On-Board Diagnostics Generation-II (OBD-II) Systems
On-Board Diagnostics OBD I
CARB 1st OBD regulation
OBD Gen I (OBD I)
All vehicles sold in
CA 1988
Warning lamp
Transmit DTCs
Electronic system monitoring of the HO2S, EGR valve, and evaporative purge solenoid
FIGURE 10-1

4. On-Board Diagnostics Generation-II (OBD-II) Systems
OBD-II Objectives
CARB defines an OBD-II-equipped to do following:
Detect component degradation
Faulty emission-related system that prevents compliance with federal emission standards.
Alert driver of needed emission-related repair or maintenance.
Standardized DTCs, accept a generic scan tool
1996 & later vehicles

5. On-Board Diagnostics Generation-II (OBD-II) Systems
Clean Air Act of 1990
EPA develop new regulations for OBD
OBD II Primary purpose emission reduction
OBD I primary purpose of OBD I (1988) detect faults
OBD-II regulations require sensors be tested & all exhaust emission control devices
Verified for proper operation.
New vehicles must pass Federal Test Procedure (FTP)

6. On-Board Diagnostics Generation-II (OBD-II) Systems
Federal Test Procedure (FTP)
Exhaust emissions tested 505 seconds, rollers
Simulate urban drive cycle around downtown LA
IM 240 a 240-second version of 505-second FTP
Exhaust emissions are within 1.5 times the FTP limits
Computer must do the following:
Test all exhaust emission system components
Actively operate the system and measure the results.
Continuously monitor emissions do not exceed 1.5 X FTP
Check engine operation for misfire
Turn on MIL PCM senses a fault
Record a freeze frame
Flash MIL if an engine misfire occurs

7. Diagnostic Executive PCM Incorporates Special Software
GM call it diagnostic executive.
Manages operation of all OBD-II monitors
Controlling sequence of steps
Execute diagnostic tests and monitors

8. MONITORS Organized Method of Testing Monitors
Simply tests, PCM performs to evaluate components & systems
If component, system failure detected while a monitor is running
DTC stored & MIL illuminated by second trip
Two types of monitors used: continuous and noncontinuous

9. MONITORS Continuous Monitors Conditions met
Continuous monitors begin to run
Run for remainder of vehicle drive cycle
3 continuous monitors are as follows:
Comprehensive component monitor (CCM)
Misfire monitor
Fuel trim monitor

10. MONITORS Comprehensive Component Monitor (CCM)
Watches sensors & actuators
Sensor values are constantly compared
Known-good values stored in PCM memory
CCM, internal program in PCM
Monitor
Failure in any electronic component or circuit
Provides input or output signals to the PCM

11. MONITORS Misfire Monitor Looks at engine misfire
PCM uses information received
From crankshaft position sensor (CPK)
Calculate time between edges of reluctor
As well as rotational speed and acceleration
Comparing acceleration of each firing event
PCM determines if a cylinder is not firing correctly
Misfire type A
Misfire type B

12. MONITORS Fuel Monitor PCM monitors short & long term fuel trim
Constantly updated adaptive fuel tables are stored in long-term memory (KAM)
PCM uses data to compensate for wear & aging of fuel system components
MIL illuminates when PCM determines fuel trim values
Reached and stayed at their limits for too long a period

13. MONITORS Noncontinuous Monitors Run once per vehicle drive cycle
Noncontinuous monitors are:
O2S monitor & O2S heater monitor
Catalyst monitor
EGR monitor
EVAP monitor
Secondary AIR monitor
Transmission monitor
PCV system monitor
Thermostat monitor

14. Diagnostic Trouble Code Priority
CARB Mandated All DTCs
Stored according to individual priority
DTCs with a higher priority overwrite lower priority
OBD-II System DTC Priority include:
Priority 0 – Non-emission related codes
Priority 1 – One-trip failure of two-trip fault for non-fuel, non-misfire codes
Priority 2 – One-trip failure of two-trip fault for fuel or misfire codes
Priority 3 – Two-trip failure or matured fault of non-fuel, non-misfire codes
Priority 4 – Two-trip failure or matured fault for fuel or misfire codes

15. OBD-II Monitor Information Summary
Comprehensive Components Monitor (CCM)
Circuits & components covered by CCM
Do not include those directly monitored by another monitor
OBD II requires inputs from powertrain components to PCM
Tested for rationality, & outputs to powertrain components from PCM
Tested for functionality

16. OBD-II Monitor Information Summary
Continuous Running Monitors
Monitors run continuously, stop if they fail
Examples:
Fuel system: rich/lean
Misfire: catalyst damaging/FTP (emissions)
Two-trip faults (except early gen catalyst damaging misfire)
MIL, DTC, freeze frame after two consecutive faults
Freeze frame is priority 2 on first trip
Freeze frame is priority 4 on maturing trip
3 consecutive good trips in a similar condition window to are used to extinguish the MIL

17. OBD-II Monitor Information Summary
Once Per Trip Monitors
Monitor runs once per trip, pass or fail
O2 response, O2 heaters, EGR, purge flow EVAP leak, secondary air, catalyst
Two-trip DTCs
MIL, DTC, freeze frame after 2 consecutive faults
Freeze frame is priority 1 on first trip
Freeze frame is priority 3 on maturing trip
3 consecutive good trips are used to extinguish the MIL
40 warm-up cycles used to erase DTC & freeze frame

18. OBD-II Monitor Information Summary
Exponentially Weighted Moving Average (EWMA)
Monitors
Catalyst monitor
EGR monitor
PCM runs 6 consecutive failed tests; fails in 1-trip
3 consecutive failed tests on next trip, then fails
Freeze frame is priority 3
3 consecutive good trips are used to extinguish the MIL
40 warm-up cycles are used to erase DTC and freeze frame

19. ENABLING CRITERIA Diagnostic Executive (Task Mgr) Enabling Criteria
Internal director to keep track
When each monitor should run
Each monitor has enabling criteria
Conditions must be met
Before task manager will give go-ahead for each monitor to run

20. Monitor Running Conditions
Overview
Monitor running conditions required
Diagnostic monitor to run.
E.G.: Specific RPM, ECT, MAP, run time, VSS, etc.

21. Monitor Running Conditions
Pending
Some situations PCM will not run a monitor
MIL is illuminated
Fault is stored from another monitor
PCM postpones monitors pending resolution of original fault
PCM does not run the test until the problem is remedied

22. Monitor Running Conditions
Conflict
PCM does not run a monitor
Another monitor is in progress
Effects of another monitor running result in erroneous failure
Conflict present, monitor not run until conflicting condition passes
Monitor will run later after conflicting monitor has passed

23. Monitor Running Conditions
Suspend
PCM may not allow a two-trip fault to mature
PCM will suspend the maturing fault
If a condition exists that may induce erroneous failure
Prevents illuminating MIL for wrong fault
Allows more precise diagnosis

24. Monitor Running Conditions
Rationality Test
PCM constantly monitors
Inputs for electrical opens and shorts
Tested for rationality
Means input signal is compared
Against other inputs and information
See if it makes sense under current conditions

25. Monitor Running Conditions
FREQUENTLY ASKED QUESTION
What Does “Rationality Check” Mean?
PCM programmed to detect faults
Do not seem rational
Engine operating for 20 minutes coolant temp changes
195°F to -40°F
Determines not possible (rational) defaults to fail-safe temp
Before OBD-II regulations, increase fuel delivered
OBD-II computer can reason that there must be a fault
Continue to deliver fuel for proper operation
Not too much that could affect the exhaust emissions

26. OBD-II Drive Cycle Trip Key-On/Key-Off condition
Key-on, conditions for a particular test
To be performed followed by a key off
Conditions called enable criteria
EGR test engine at normal operating temperature
Decelerating for a minimum amount of time

27. OBD-II Drive Cycle Warm-Up Cycle
Trip with engine temperature increase of
At least 40°F
Engine temperature reaches 160°F (71°C).
E.G.: MIL is deactivated
Original code remains in memory until 40 warm-up cycles are completed
Without fault reappearing

28. OBD-II Drive Cycle MIL Condition: Off PCM has not detected any faults
Emissions-related component or system
MIL circuit is not working

29. OBD-II Drive Cycle MIL Condition: On Steady
Condition indicates a fault
Emissions-related component
System that could affect vehicle emission levels

30. OBD-II Drive Cycle MIL Condition: Flashing Condition indicates
Misfire or fuel control system fault
That could damage catalytic converter

31. OBD-II Drive Cycle MIL Off PCM turns off the MIL
Following actions or conditions occur:
Codes cleared with scan tool
Power to PCM removed for an extended period of time
Driven on 3 consecutive trips with a warm-up cycle
Meets code set conditions without PCM detecting faults

32. OBD-II DTC Numbering Designation
DLC
Data Link Connector
FIGURE 10-2

33. OBD-II DTC Numbering Designation
Diagnostic Trouble Code (DTC)
Scan tool required, retrieve DTCs
Every OBD-II scan tool
Read
Generic SAE DTCs
Major categories:
Pxxx codes – powertrain DTCs
Bxxx codes – body DTCs
Cxxx codes – chassis DTCs
Uxxx codes – network DTCs
FIGURE 10-3

34. OBD-II DTC Numbering Designation
DTC Numbering Explanation
Number in hundredth position
Indicates specific vehicle system
Subgroup that failed.
Consistent for P0xxx and P1xxx type codes
Example: P0100 – Air metering and fuel system fault

35. OBD-II DTC Numbering Designation
FREQUENTLY ASKED QUESTION
Is There a Generic Drive Cycle?
Readiness tests (sometimes called flags)
Tests performed on all of the monitored systems on Tech2
Published drive cycles for each OEM
Generic drive cycle usually allows all monitors to run
Coolant temperature < 122°F (50°C) IAT within 11°F (6°C)
ECT temperature
Fuel tank filled from 15% to 85% capacity before test

36. OBD-II DTC Numbering Designation
Types of DTCs
Not all OBD-II DTCs
Same importance
Each type of DTC has different requirements

37. OBD-II DTC Numbering Designation
Type A Codes
Type “A” DTC is emission related
MIL turned on first trip
PCM detected a problem.
Engine misfire or a very rich or lean air-fuel

38. OBD-II DTC Numbering Designation
Type B Codes
B code stored, MIL turned
During 2nd consecutive trip
Alerting driver to fact
Diagnostic test was performed and failed

39. OBD-II DTC Numbering Designation
Type C and D Codes
Non-emission-related diagnostic tests
Cause lighting of MIL lamp
Vehicle is so equipped
Type C codes are also called
Type C1 codes and D codes are also called type C0 codes

40. OBD-II DTC Numbering Designation
OBD-II Freeze Frame
OBD II requires PCM
Take a "snapshot" or freeze frame
All data at instant an emission-related DTC is set
Scan tool (Tech 2) required

41. OBD-II DTC Numbering Designation
NOTE:
OBD II requires one freeze frame
Data be stored
Instant an emission-related DTC is set
GM provides expanded data about DTC
Beyond that required
Retrieving this enhanced data
Requires Tech 2 vehicle-specific scan tool

42. OBD-II DTC Numbering Designation
Freeze Frame
Items:
Listed below
Calculated load value & RPM
Short-term and long-term fuel trim percent
Fuel system pressure (on some vehicles)
Vehicle speed (mph)
Engine coolant temperature
Intake manifold pressure
Closed/open-loop status
Fault code that triggered the freeze frame
If a misfire code is set, identify cylinder misfiring

43. FREEZE FRAME DTC Should Not Be Cleared From vehicle computer memory
Unless corrected
OBD II requires snap shot of engine parameters
Instant a DTC was set
Help determine what caused the fault
Help verify that the fault has been corrected
Problem corrected, PCM automatically clears DTC
After 40 consecutive warm-up cycles no faults detected
Misfire, rich or lean condition codes require 80 cycles
Codes can also be erased by using scan tool

44. FREEZE FRAME NOTE: Disconnecting battery May not erase OBD-II DTCs
Or freeze frame data
GM recommends using Tech 2 to erase DTCs
Rather than disconnecting battery
Memory for the radio, seats
Learned engine operating parameters
Lost if the battery is disconnected

45. FAILURE RECORDS Enhance Freeze Frame Data Required by OBD II
GM also provide additional information
Called failure records
Data included in failure records
Allow technician to see more about operating conditions
Vehicle was experiencing when failure occurred

46. FAILURE RECORDS REQUENTLY ASKED QUESTION What Are Pending Codes?
Pending codes are set operating conditions met
Component or circuit is not within the normal range
Conditions have not yet been met to set a DTC
Sensor may require 2 consecutive faults before DTC set
Scan tool displays a pending code or a failure
Driveability concern could also be present
Pending code can help technician determine root cause
Before customer complains of MIL

47. NEED for FUEL TRIM Fuel Trim Purpose Provide catalytic converter
Rich and a lean air-fuel mixture
Rich mixture (lack of extra oxygen) needed
Reduce NOX exhaust emissions
Lean mixture (excessive oxygen) needed
Oxidize HC and CO into harmless
Carbon dioxide (CO2) and water (H2O) vapor

48. NEED for FUEL TRIM Exhaust Always Rich
Catalytic converter cannot reduce
CO and HC emissions
Exhaust is always lean, catalytic converter
Cannot reduce NOX emissions
Air-fuel mixture must alternate between rich and lean
PCM provides this alternating mixture
Using oxygen sensor, short-term and long-term fuel trim
Program to accomplish this feat

49. SHORT-TERM FUEL TRIM STFT % Measurement
Amount PCM adding or subtracting
EFI systems use O2S
Determine whether exhaust is rich or lean
Without O2S, control of fuel delivery
Belongs to PCM alone using programmed PWM commands
Based on other sensor inputs
ECT, TPS, and MAP

50. SHORT-TERM FUEL TRIM Operating in Closed Loop
O2S signal can modify or change
Preprogrammed fuel delivery
Fuel trim is expressed as a percentage (%), (+) or (-)
Amount of fuel different from anticipated amount
Small vacuum leak were to occur
O2S produces lower voltage signal interpreted by PCM
Air-fuel mixture is too lean
Pulse width increased to compensate for vacuum leak
Additional fuel added as positive short-term fuel trim

51. SHORT-TERM FUEL TRIM NOTE: Before 1996, GM referred to STFT Integrator
Expressed it in binary numbers from 0 to 255
128 was the midpoint and a reading of ±10 from 128
( ) was usually considered to be a normal reading
FIGURE 10-4

52. SHORT-TERM FUEL TRIM STFT of +20% Indicates 20% additional fuel added
Achieve proper air-fuel mixture
-20% short-term fuel trim indicates
Fuel had to be removed by shortening injector pulse width
Achieve the proper air-fuel mixture.
STFT varies air-fuel mixture so catalytic converter
Reduce HC, CO, and NOX exhaust emissions
Large vacuum leak occurs, fuel delivery must be increased
For a Longer Period
EFI computers must use a long-term fuel trim program

53. LONG-TERM FUEL TRIM LTFT: Add or Subtract Fuel Larger amount of time
Than short-term fuel trim
LTFT looked at by technician as a guide
PCM has been adding or subtracting fuel
Accomplish the proper air-fuel mixture
Vacuum hose split open, engine leaner than normal
STFT attempts to add fuel right away to adjust
Leak remained for longer than a few seconds
PCM revises to long-term fuel trim
Compensate for the leak over a larger period of time

54. LONG-TERM FUEL TRIM LTFT Makes an Adjustment
STFT can still make short and quick changes
In air-fuel mixture needed
Provide the catalytic converter with an alternating rich
Then lean, then rich, and so forth, exhaust

55. LONG-TERM FUEL TRIM REAL WORLD FIX Red S-10 Pickup Truck Story
4-cylinder 2.2-liter engine was replaced
Replacement engine never did run correctly, idle & low speeds
-25% long-term fuel trim, oxygen sensor was measuring a very rich exhaust from spent exhaust gases
Oxygen sensor indicating a false rich condition
None of the false rich conditions was found
Incorrect EGR gasket was found
Caused exhaust gases to flow into the cylinders all time

56. Fuel Trim as Diagnostic Aid
Fuel Trim Values
Only be observed with scan tool
Display both short-term and long-term fuel trim
For system diagnosis, refer to the long-term fuel trim
Because it represents a longer amount of time (history)
Greater amount of mixture correction

57. Fuel Trim as Diagnostic Aid
NOTE
STFT and LTFT
Make corrections to fuel delivered
Achieve proper air-fuel mixture
+25% LTFT, will require PCM to deliver 30% more
Calibrated amt of fuel to achieve proper air-fuel mixture
Engine is now operating with correct air-fuel mixture
LTFT # tells technician what PCM had to do to achieve
Proper mixture

58. FUEL TRIM CELLS STFTs and LTFTs React to oxygen sensor voltage
Modify fuel delivery
Most vehicles set aside different cells for
Each combination of engine speed (RPM) and load
PCM can then correct for slight differences in fuel mixture
Some GM vehicles use 16 cells
Plus 2 for deceleration and 2 for idle only

59. Fuel Trim Cell Diagnosis
Data Observed
During same condition as problem
Two cells for idle
One with air conditioning (A/C) on and one for A/C off
Customer’s concern only occurs when A/C is on
Observe fuel trim numbers on Tech 2
Engine operating at idle and with the A/C on

60. Fuel Trim Cell Diagnosis
Problem at 55 mph (90 km/h)
Looking at fuel trim in service bay
At idle will not help at all
Vehicle must be driven under similar conditions
Duplicate the condition when the problem occurs
Only then will the correct fuel cell be displayed
Long-term fuel trim information should be valid
See fuel trim diagnostic chart

61. Fuel Trim Cell Diagnosis
Fuel Trim Diagnostic Chart

62. Fuel Trim & Misfire Codes
Misfire-related code sets
Driven under conditions similar when fault detected
Before PCM will deactivate the MIL
Similar conditions are:
Engine speed within 375 RPM of speed stored in freeze frame data when the code set
Engine load ±10% of load value stored in freeze frame data when the code set
Temperature conditions similar to temperature value stored in freeze frame data when the code set

63. GENERIC OBD II All OBD-II Vehicles Must be able to display data
Generic (also called global) scan tool
Under nine different modes of operation
Information may be helpful if a generic scan tool used
Diagnose GM vehicles instead of using a TECH 2

64. GENERIC OBD II Global Scan Tool 9 modes of operation, include:
Mode One: Current powertrain data
Mode Two: Freeze frame data
Mode Three: Diagnostic trouble codes
Mode Four: Clear, reset DTCs
Mode Five: Oxygen sensor monitor test results
Mode Six: Non-continuously monitored systems
Mode Seven: continuously monitored systems
Mode Eight: bidirectional control of onboard systems
Mode Nine: module identification

65. Summary
If the MIL is on, retrieve the DTC and follow the manufacturer's recommended procedure to find the root cause of the problem.
All monitors must have the enable criteria achieved before a test is performed.
OBD-II vehicles use a 16-pin DLC and common DTCs.
OBD II includes generic (SAE), as well as, vehicle manufacturer’s specific DTCs and data display.

66. Review Questions
What does the PCM do during a drive cycle to test emission-related components?
What is the difference between a type A and type B OBD-II DTC?
What is the difference between a trip and a warm-up cycle?
What could cause the MIL to flash?

67. FIGURE 10-3 

68. FIGURE 10-4 

69. TECH TIP
Don’t Forget - Three Clicks: OBD II requires that fuel system integrity be checked for possible leakage. If fuel (gas) cap is not securely tightened, then a DTC such as P0442 may be set. To help prevent such false codes and to ensure that gas cap is properly tightened, GM has printed on the cap itself a note that the cap should be tightened until three clicks are heard. This also applies to ALL screw-thread-type gas caps. NOTE: Gas caps are frequently tested as part of an exhaust emission test. Ask person performing the test on your gas cap to tighten the cap 3 clicks to ensure a proper tightness. This will help prevent false defective test results. 

70. TECH TIP
Think of a Small Faucet and a Large Faucet: The purpose of fuel trim is to add or subtract fuel as needed to maintain the proper air-fuel mixture so the catalytic converter can operate properly. STFT is fast but can add or subtract a small amount of fuel. This can be visualized as being similar to a small water faucet adding water to a sink. For example, if a small vacuum hose were to become disconnected, the STFT will add a little extra fuel to compensate for the added amount of air being drawn into the engine. 