Things You'll Need:
- OBD scanner for your vehicle
- Various automotive tools
- Scan Tool
-
Step 1
5-GAS DIAGNOSIS
Is the ECM in fuel control? (closed loop, you'll need a scantool to verify this)
Check O2 Sensor operation, ECM codes and data, ECM inputs and outputs, ECM
Is the ECM now in fuel control
NO
Re-inspect items above
Retest Vehicle
Did it pass
NO
What Failed
Failed HC go to HC
Failed CO go to CO
Failed NOx go to NOx
Failed HC & CO go to CO
Failed HC & NOx go to HC
Failed HC, CO & NOx go to CO -
Step 2
DIAGNOSING FAILED HC (hydrocarbons)
Inspect for any cracked, loose, broken vacuum fittings or vacuum leaks
Inspect ignition system (wires, rotor, cap, etc.) for obvious wear
Inspect ignition timing and base engine settings including idle and valve adjustment
Inspect ECM for codes and correct data
Inspect ignition system primary and secondary kV within specification?
Inspect fuel system pressure, blockage or leaks (carb or injectors), proper operation
Inspect engine manifold vacuum
Vacuum should be between 16-20in. Hg at idle and steady or within manufacturer's specifications
Inspect engine mechanical
Perform compression test (cranking and running)
Inspect Catalytic Converter
Is post-cat HC reading less than 225 ppm?
(IF ANY OF THE ABOVE QUESTION ANSWERED NO, REPAIR AND RETEST VEHICLE)
HC less than 225 ppm post-catalyst
Perform a pre-cat test to verify cat effeicency and that the air/fuel mixture is in stoichiometry
Is pre-cat HC reading less that 225 ppm?
NO
HC greater than 225 ppm pre & post-catalyst
Verify ECM is in proper fuel control
Repeat all HC diagnostic steps
Perform engine de-carbonizing
Verify catalyst efficiency after repairs
HC less than 225 ppm pre-cat
(note: verify A.I.R. system is operating correctly, if equipped)
Replace catalyst and retest vehicle
Catalyst has degraded and is no longer capable of oxidizing HC or CO -
Step 3
DIAGNOSING FAILED CO (carbon monoxide)
Inspect for any cracked, loose, broken vacuum fittings or vacuum leaks
Inspect ignition system (wires, rotor, cap, etc.) for obvious wear
Inspect ignition timing and base engine settings including idle and valve adjustment
Inspect ECM for codes and correct data
Inspect ignition system primary and secondary kV within specification?
Inspect fuel system pressure, blockage or leaks (carb or injectors, EVAP), proper operation
Inspect engine manifold vacuum
Vacuum should be between 16-20in. Hg at idle and steady or within manufacturer's specifications
Inspect engine mechanical
Perform compression test (cranking and running)
Inspect Catalytic Converter
Is post-cat CO reading less than 1.5%?
(IF ANY OF THE ABOVE QUESTION ANSWERED NO, REPAIR AND RETEST VEHICLE)
CO less than 1.5% post-cat
Perform a pre-cat test to verify catalyst efficiency and that the air-fuel mixture is in stoichiometry (ratio of 14.7 air to 1.0 fuel)
Is pre-cat CO reading less than 1.5%
NO
CO greater than 1.5% pre & post-catalyst
Verify ECM is in proper fuel control
Repeat all CO diagnostic steps
Perform engine de-carbonizing
Verify catalyst efficiency after repairs
CO less than 1.5% pre-catalyst
Replace catalyst & retest vehicle
(note: verify A.I.R. system is operating correctly, if equipped)
Catalyst has degraded and is no longer capable of oxidizing HC or CO
CO less than 0.75% pre-cat indicates that the ECM is in fuel control; reverify ECM fuel control if the CO reading was above 1.0% -
Step 4
DIAGNOSING FAILED NOx A (oxides of nitrogen)
Inspect for any cracked, loose, broken vacuum fittings or vacuum leaks
Inspect ignition system (wires, rotor, cap, etc.) for obvious wear
Inspect ignition timing and base engine settings including idle and valve adjustment
Inspect EVAP system, if equipped
Inspect the Catalyst
Perform a pre-cat test to verify catalyst efficiency and that the air/fuel mixture is in stoichiometry (ratio of 14.7 air to 1.0 fuel)
Is there a difference of at least 1000 ppm or 45% in the pre & post-catalyst NOx reading?
Is vehicle equipped with an EGR system?
Inspect EGR system
Record NOx readings pre-cat, then disable the EGR and record NOx readings again
Compare the two pre-cat NOx readings
Is there an increase of at least 45% or 1000 ppm NOx pre-cat?
Inspect the following EGR system components
passages, valve, vacuum regulators or solenoids, vacuum hoses & routing, exhaust system back pressure
Repair and retest vehicle with EGR disabled and then enabled; compare both readings
Is there and increase of at least 45% or 1000 pmm NOx pre-cat?
NO
Re-Inspect all items and retest EGR system
YES
Go to NOx Steps B
NOx reduction less than 45% or 1000 ppm
Is O2 content below 1% pre-cat?
YES
Go to NOx Steps C
O2 above 1.0% pre-cat
Is vehicle equipped with A.I.R. system?
Vehicle equipped with A.I.R. system
Is A.I.R. system pumping air upstream of catalyst
YES
Repair all items and retest vehicle
No
Inspect the following
exhaust system for dilution (leaks)
proper fuel control (lean running condition)
recheck oxygen percentage level
is O2 level below 1.0% pre-cat?
NO
Re-inspect all times and re-test O2 level
YES
Go To NOx Steps C -
Step 5
DIAGNOSING FAILED NOx B (oxides of nitrogen)
Inspect ECM for codes and data
Inspect engine coolant operating temperature data
Inspect knock retard or computer controlled timing & dwell data parameters, if equipped
Inspect all NOx related data parameters including air temp & EGR control
Inspect engine temperature
Inspect thermostat cycling properly
Is coolant flow and airflow adequate at idle and cruising speeds
(tip: use infrared gun on all header pipes to locate lean cylinders or overheating conditions)
Inspect ignition timing
Is ignition timing & dwell with specifications at idle and under load?
Inspect engine mechanical
Perform compression test (cranking and running)
Is compression with specifications?
Inspect fuel system
Inspect fuel system pressure
Inspect for clogged fuel injectors
Is fuel system operating properly?
NO
Repair all items & re-test vehicle
YES
Repeat NOx Diagnostic Steps
Perform engine de-carboning -
Step 6
DIAGNOSING FAILED NOx C (oxides of nitrogen)
O2 less than 1.0% pre-cat
Verify ECM is in proper fuel control
Perform engine de-carboning
Re-test vehicle
Did the vehicle pass for NOx?
YES - testing complete
NO
NOx reduction less than 45% or 1000 ppm
Replace catalyst and re-test vehicle
Catalyst has degraded and no longer has the ability to efficiently reduce NOx
Replace Catalyst and Re-test Vehicle
TESTING PROCEDURES ARE COMPLETE
