Diagnostic Aids

Perform this test when instructed during QUICK TEST or if directed by other test procedures. This test is used to diagnose the exhaust system and downstream HO2S.

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.

NOTE: If directed to this test for symptoms only, go to step 5).

1) Continuous Memory DTC P0420 Or P0430: Check For Misfire Detection Monitor DTCs

DTC P0420 indicates bank one catalyst system efficiency is below acceptable threshold. DTC P0430 indicates bank 2 catalyst system efficiency is below acceptable threshold. Possible causes are:

Use Of Leaded Fuel
Faulty HO2S
Faulty ECT Sensor
High Fuel Pressure
Damaged Exhaust System Component
Oil Contamination
Cylinder Misfire
Downstream HO2S Sensor Improperly Connected
Retarded Spark Timing

Retrieve all Continuous Memory DTCs. Check for any misfire detection monitor DTCs recorded. See MISFIRE DIAGNOSTIC TROUBLE CODES table. If no misfire codes are present, go to next step. If any misfire codes are present, isolate cylinder and perform appropriate system test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .

MISFIRE DIAGNOSTIC TROUBLE CODES

Misfire DTC

Cylinder

P0300

⁽¹⁾

P0301

No. 1

P0302

No. 2

P0303

No. 3

P0304

No. 4

P0305

No. 5

P0306

No. 6

P0307

No. 7

P0308

No. 8

P0309

No. 9

P0310

No. 10

⁽¹⁾	Multiple cylinder misfire or PCM cannot identify which cylinder is misfiring.

2) Check For HO2S Monitor DTCs

If DTC P0136, P0138, P0141 (bank one, rear HO2S), P0156, P0158 or P0161 (bank 2, rear HO2S) were present in step 1) , perform appropriate system test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS . If none of these DTCs were present in step 1) , go to next step.

NOTE: ECT or CHT sensor DTCs can indicate thermostat is not operating correctly or engine coolant level is not filled to specification, possibly causing higher than normal operating temperatures.

3) Check For ECT Or CHT Sensor DTCs
If DTC P0117, P0118, P0125, P1117, P1285, P1288, P1289, P1290 or P1299 was present in step 1) , perform appropriate system test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS . If none of these DTCs are present in step 1) , go to next step.

4) Check For Any Other DTCs

If any other DTCs except P0420 and P0430 are present in step 1) , perform appropriate system test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no other DTCs except P0420 and/or P0430 are present in step 1) , go to next step.

5) Check Rear HO2S Wiring & PCM Connections

Turn ignition switch to OFF position. Ensure HO2S wiring harness is correctly routed, wires are not crossed and connectors are tight. Repair as necessary. Disconnect PCM connector(s). Inspect connector for loose, damaged or corroded terminals. Repair as necessary. Check PCM terminals. If PCM terminals are damaged, replace PCM. Perform DRIVE CYCLE PROCEDURES under ADDITIONAL SYSTEM FUNCTIONS. Ensure CATALYST MONITOR PROCEDURE is performed to verify repair. If wiring harness, connectors and terminals are okay, go to next step.

6) Check Fuel Pressure

Turn ignition switch to OFF position. Release fuel pressure. See FUEL SYSTEM PRESSURE RELEASE procedure under FUEL SYSTEM in appropriate REMOVAL, OVERHAUL & INSTALLATION article. On vehicles with returnable fuel systems, check vacuum hose and vacuum supply to fuel pressure regulator. Repair as necessary. On vehicles with Mechanical Returnless Fuel Systems (MRFS), check vacuum hose and vacuum supply to fuel rail pulse damper. Repair as necessary. Connect scan tool to Data Link Connector (DLC). On vehicles with Electronic Returnless Fuel Systems (ERFS), monitor fuel pressure by selecting FRP PID from PID/DATA MONITOR & RECORD menu. On all other models, install fuel pressure gauge. Start engine and allow to idle. Record FRP PID value or fuel pressure gauge reading at idle. Increase and hold engine speed to 2500 RPM for one minute. Record FRP PID value or fuel pressure gauge reading with engine speed increased. Fuel pressure should be within specification. See FUEL PRESSURE SPECIFICATIONS table. If fuel pressure is as specified, go to next step. If fuel pressure is not as specified, go to TEST HC, step 4).

7) Check For Obvious Exhaust System Leaks

If exhaust system leaks, it may cause CATALYST MONITOR to fail. Inspect complete exhaust system for cracks, loose connections or punctures. Repair as necessary. Perform DRIVE CYCLE PROCEDURES under ADDITIONAL SYSTEM FUNCTIONS. Ensure CATALYST MONITOR PROCEDURE is performed to verify repair. If exhaust system is okay, go to next step.

8) Check For Obvious Exhaust System Restrictions

Inspect complete exhaust system for collapsed areas, dents or excessive bends. Repair or replace as necessary. Perform DRIVE CYCLE PROCEDURES under ADDITIONAL SYSTEM FUNCTIONS. Ensure CATALYST MONITOR PROCEDURE is performed to verify repair. If exhaust system is okay, go to next step.

9) Check Manifold Vacuum For Indication Of Excessive Exhaust System Restriction

Install tachometer. Connect vacuum gauge to intake manifold vacuum source. Place transmission in Nuetral. Start engine and gradually raise engine speed to 2000 RPM. Manifold vacuum reading should increase to more than 16 in. Hg. Allow engine speed to return to idle. Turn ignition switch to OFF position. If manifold vacuum reading was more than 16 in. Hg, go to next step. If manifold vacuum reading was less than 16 in. Hg, go to step 11).

10) Check Manifold Vacuum For Indication Of Moderate Exhaust System Restriction

Leave tachometer and vacuum gauge connected. Start engine and gradually raise engine speed to 2000 RPM. Observe vacuum gauge reading response while maintaining increased engine RPM. If an exhaust system is free of restrictions, manifold vacuum increases quickly to normal range as increased RPM is maintained. If exhaust system is restricted, manifold vacuum increases slowly to normal range as increased 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. If directed here for DTCs P0420 or P0430, catalytic converter is chemically inactive. Replace catalytic converter. Perform DRIVE CYCLE PROCEDURES under ADDITIONAL SYSTEM FUNCTIONS. Ensure CATALYST MONITOR PROCEDURE is performed to verify repair. If directed here for symptom diagnosis, (lack of power, loss of power, no start, etc.), return to appropriate symptom under SYMPTOMS in TROUBLE SHOOTING - NO CODES article to complete testing.

NOTE: An intake manifold gasket leak can cause vacuum gauge reading to remain below normal range.

11) Check Manifold Vacuum With Exhaust System Disconnected From Exhaust Manifold
Leave tachometer and vacuum gauge connected. Disconnect exhaust pipe from exhaust manifold. Start engine and gradually raise engine speed to 2000 RPM. Observe vacuum gauge reading response while maintaining increased engine RPM. If manifold vacuum is okay, exhaust restriction is downstream of exhaust manifold. Reconnect exhaust system to exhaust manifold and go to next step. If manifold vacuum is low or slow to respond, restriction is present in exhaust manifold. Check exhaust manifold for restrictions. If restrictions are present and are unable to be removed, replace exhaust manifold. Perform DRIVE CYCLE PROCEDURES under ADDITIONAL SYSTEM FUNCTIONS. Ensure CATALYST MONITOR PROCEDURE is performed to verify repair.

12) Check Manifold Vacuum With Muffler & Tailpipe Asembly Disconnected

Leave tachometer and vacuum gauge connected. Disconnect muffler and tailpipe assembly from rear of catalytic converter. Start engine and gradually raise engine speed to 2000 RPM. Observe vacuum gauge reading response while maintaining increased engine RPM. If manifold vacuum is now okay, fault is in muffler and tailpipe assembly. Repair as necessary. Perform DRIVE CYCLE PROCEDURES under ADDITIONAL SYSTEM FUNCTIONS. Ensure CATALYST MONITOR PROCEDURE is performed to verify repair. If manifold vacuum is still not okay, fault is in catalytic converter. Replace catalytic converter. After replacing catalytic converter, check muffler and tailpipe assembly for debris from faulty catalytic converter. Perform DRIVE CYCLE PROCEDURES under ADDITIONAL SYSTEM FUNCTIONS. Ensure CATALYST MONITOR PROCEDURE is performed to verify repair.