HF7 CHECK FOR OBVIOUS LEAK SOURCES IN THE EXHAUST SYSTEM
Note: If a catalyst is in series with a leaking exhaust system, it can fail the Catalyst Efficiency Monitor test.

Key off.
Inspect the following for leaks, cracks, loose connections or punctures:
Exhaust manifold.
Front exhaust pipe.
Rear exhaust pipe.
Muffler/tail-pipe assembly.
Are the above components free of cracks and punctures, etc.?
Yes No
CHECK that the exhaust manifold to catalyst inlet pipe joint is tight. GO to HF8 . REPLACE/REPAIR the leak source(s). COMPLETE PCM Reset to clear DTCs (REFER to Section 2A, Powertrain Control Module (PCM) Reset ). RERUN Quick Test .

HF8 CHECK FOR OBVIOUS RESTRICTIONS IN THE EXHAUST SYSTEM
Inspect the following for dents, areas of collapsed material and unusual bending:
Front exhaust pipe.
Rear exhaust pipe.
Muffler/tail-pipe assembly.
Are the components free of dents and areas of collapsed material or unusual bending, etc.?
Yes No
GO to HF9 . REPLACE/REPAIR the restricted component(s) as necessary. COMPLETE PCM Reset to clear DTCs (REFER to Section 2A, Powertrain Control Module (PCM) Reset ). RERUN Quick Test .

HF9 CHECK MANIFOLD VACUUM FOR INDICATION OF EXCESSIVE EXHAUST SYSTEM RESTRICTION
Attach a vacuum gauge to the intake manifold vacuum source.
Install a tachometer.
Observe the vacuum gauge needle while performing the following:
Note: The vacuum gauge reading may be normal when the engine is first started and idled. However, excessive restriction in the exhaust system will cause the vacuum gauge needle to drop to a low point even while the engine is idled.

Start the engine and gradually increase the rpm to 2000 with the transmission in NEUTRAL.
Decrease engine speed to base idle rpm.
Key off.
Did manifold vacuum rise above 16 inches Hg with the engine rpm at 2000?
Yes No
GO to HF10 . Manifold vacuum did not reach an acceptable level. An excessive restriction may be present. GO to HF11 .

HF10 CHECK MANIFOLD VACUUM FOR INDICATION OF MODERATE EXHAUST SYSTEM RESTRICTION
Vacuum gauge installed.
Tachometer installed.
Key on, engine idling.
Increase the engine speed gradually from base idle rpm to 2000 rpm with the transmission in NEUTRAL.
Observe the speed the vacuum gauge needle rises, while maintaining the increased engine rpm.
Note: 1: On a non-restricted exhaust system, the vacuum gauge needle will rise quickly to the normal range as the increased rpm is maintained.

Note: 2: On a restricted exhaust system, the vacuum gauge needle will rise slowly to the normal range as the increased rpm is maintained.

Note: 3: The rate of speed the vacuum gauge needle rises to the normal range is slower on a restricted system than on a non-restricted system as the increased rpm is maintained.

Decrease engine speed to base idle rpm.
Key off.
Is the rate of speed that the vacuum gauge needle rises back to the normal range (above 16 inches Hg) much slower than that of a non-restricted system?
Yes No
A moderate restriction may be present. GO to HF11 . No indications of restrictions or leaks have been detected in the exhaust system. If here because of DTCs P0420/P0421 or P0430/P0431, the catalytic converter is chemically inactive. REPLACE the catalytic converter, being careful to avoid improper routing of the rear HO2S. COMPLETE PCM Reset to clear DTCs (REFER to Section 2A, Powertrain Control Module (PCM) Reset ). RERUN Quick Test . REMOVE the vacuum gauge and tachometer. For further diagnosis of symptom (e.g. Lack of Power, Loss of Power, or No Start) REFER to Section 4A, Symptom Flowcharts.

HF11 CHECK MANIFOLD VACUUM WITH EXHAUST MANIFOLD DISCONNECTED FOR INDICATION OF A RESTRICTION
Key off.
Disconnect exhaust system immediately after the exhaust manifold.
Repeat the vacuum measurement found in test step HF10 .
Did the vacuum needle QUICKLY rise above 16 inches Hg with the engine rpm at 2000?
Note: An intake manifold gasket leak can also cause the vacuum gauge needle to remain well below the normal range.

Yes No
The exhaust system restriction is downstream of the exhaust manifold. RECONNECT exhaust system at exhaust manifold. GO to HF12 . A restriction is present in the exhaust manifold. REMOVE the vacuum gauge and tachometer. INSPECT each exhaust port for casting flash/restrictions by dropping a length of chain into it (NOTE: Do not use a wire or lamp to check the ports. The restriction may be large enough for both to pass through, but small enough to cause excessive back pressure at high engine rpm.). REPLACE the exhaust manifold if unable to remove the casting flash/restriction. COMPLETE PCM Reset to clear DTCs (REFER to Section 2A, Powertrain Control Module (PCM) Reset ). RERUN Quick Test .

HF12 CHECK MANIFOLD VACUUM WITH MUFFLER/TAIL-PIPE ASSEMBLY DISCONNECTED FOR INDICATION OF A RESTRICTION
Key off.
Disconnect muffler/tail-pipe assembly located after the catalytic converter.
Repeat the vacuum measurement found in test step HF10 .
Did the vacuum needle QUICKLY rise above 16 inches Hg with the engine rpm at 2000?
Yes No
There is a restriction in the muffler/tail-pipe assembly. REPLACE the muffler/tail-pipe assembly. REMOVE the vacuum gauge and tachometer. COMPLETE PCM Reset to clear DTCs (REFER to Section 2A, Powertrain Control Module (PCM) Reset ). RERUN Quick Test . There is a restriction in the catalytic converter. REMOVE the vacuum gauge and tachometer. REPLACE the catalytic converter, being careful to avoid improper routing of the rear HO2S and INSPECT the muffler to be certain converter debris has not entered. RECONNECT muffler/tail-pipe assembly. COMPLETE PCM Reset to clear DTCs (REFER to Section 2A, Powertrain Control Module (PCM) Reset ). RERUN Quick Test .