Diagnostic Aids

Perform this test when instructed during QUICK TEST or if directed by other test procedures. This test is used to diagnose the following:

• Exhaust system pipes.
• Muffler and tailpipe.
• Catalytic converter.
• Exhaust manifold.
• Heated Oxygen Sensor (HO2S) circuit.

Internal damage of a catalytic converter is usually caused by abnormal engine operation upstream of catalyst. Conditions that produce higher than normal temperatures in the catalytic converter, such as cylinder misfire, are likely suspects.

1. DTC P0420: Check Possible Cause Of Misfire
   
   DTC P0420 indicates catalyst system efficiency is below minimum requirement. Possible causes are as follows:
   • Use of leaded fuel.
   • Oil contamination.
   • Cylinder misfire.
   • Fuel pressure too high.
   • HO2S sensor improperly connected.
   • Damaged exhaust system component.
   • Faulty ECT sensor.
   • Faulty HO2S.

   Ensure ignition timing is correct. Retrieve all DTCs. If misfire codes are not present, go to next step. If misfire code(s) is present, isolate cylinder and repair as necessary. See TEST HD .

2. Check ECT Sensor DTCs
   
   If DTC P0115 is present, go to TEST DA . If DTC P0115 is not present in step  1, go to next step.
3. Check For Other DTCs
   
   If any DTCs except P0420 were present in step  1, service as necessary before continuing. See DIAGNOSTIC TROUBLE CODE DEFINITION CHARTS . If no DTCs except P0420 were present in step  1, go to next step.
4. Check Fuel Pressure
   
   Perform fuel delivery system diagnostic procedure. See TEST HC . If fuel delivery is okay, go to next step. If fuel delivery is not okay, repair as necessary.
5. Check For Exhaust System Leaks
   
   If exhaust system leaks, it may cause catalyst monitor efficiency test to fail. Inspect entire exhaust system for cracks, loose connections or holes. Repair or replace as necessary. If exhaust system is okay, go to next step.
6. Check For Exhaust System Restrictions
   
   Inspect exhaust system for collapsed areas, dents, excessive bending or damaged muffler. Repair or replace as necessary. If exhaust system is okay, go to next step.
7. Check Manifold Vacuum
   
   Connect vacuum gauge to intake manifold vacuum source. Connect New Generation Star (NGS) tester to DLC. Using NGS tester, access RPM PID. Observe vacuum gauge needle while starting engine and raising engine speed to 2000 RPM. Decrease engine speed to base idle RPM. Manifold vacuum should rise to greater than 16 in. Hg at 2000 RPM. If manifold vacuum is okay, go to next step. If manifold vacuum is low, go to step  9.
8. Check Manifold Vacuum For Indication Of Moderate Exhaust Restriction
   
   Leave vacuum gauge and NGS tester connected. Access RPM PID. Start and increase engine speed to 2000 RPM. On a non-restricted system, manifold vacuum should quickly increase to normal range (about 16 in. Hg) as high RPM is maintained. On a restricted system, manifold vacuum will slowly increase to normal range as high RPM is maintained. If manifold vacuum is low or slow to respond, go to next step. If manifold vacuum is okay, no indication of exhaust leak or restriction has been detected and testing is complete. If no restriction is found and DTC P0420 is active, replace ineffective catalytic converter.
9. Check Manifold Vacuum With Exhaust Manifold Disconnected For Indication Of Restriction
   
   Remove exhaust pipe from exhaust manifold. Repeat exhaust system check in previous step. If manifold vacuum is now 16 in. Hg, fault is downstream from exhaust manifold. Reconnect exhaust pipe to exhaust manifold and go to next step. If manifold vacuum is still low or slow to respond, fault is in exhaust manifold or intake manifold gasket. Repair or replace as necessary and repeat QUICK TEST .
10. Check Manifold Vacuum With Muffler/Tailpipe Disconnected For Indication Of Restriction
    
    Disconnect muffler/tailpipe assembly from rear of catalytic converter. Repeat exhaust system check in previous step. If manifold vacuum is 16 in. Hg now, fault is in muffler/tailpipe assembly. Repair or replace as necessary and test drive vehicle to verify elimination of symptom. If manifold vacuum is still not okay, fault is in catalytic converter. Repair or replace as necessary. Check tailpipe/muffler assembly for debris from catalytic converter. Test drive vehicle to verify elimination of symptom. Repeat QUICK TEST .