Diagnostic Aids

WARNING: This page does not describe the selected car, but rather 26 other vehicles, including the 1999 Mercury Tracer, 1999 Mercury Sable, 1999 Mercury Mystique, 1999 Mercury Mountaineer, and 1999 Mercury Grand Marquis. However, it is still accessible from the selected car via links, so may be relevant.

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 entering this test for symptoms only, go to step 5).

1) DTC P0420 Or P0430: Check Possible Cause Of Misfire

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

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.

Retrieve all Continuous Memory DTCs. If misfire codes are not present, go to next step. See MISFIRE DIAGNOSTIC TROUBLE CODES (DTC) table. If any misfire codes are present, isolate cylinder and repair as necessary.

MISFIRE TROUBLE CODES

Misfire DTC

Cylinder (PCM Pin No.)

P0301

No. 1 (75)

P0302

No. 2 (101)

P0303

No. 3 (74)

P0304

No. 4 (100)

P0305

No. 5 (73)

P0306

No. 6 (99)

P0307

No. 7 (72)

P0308

No. 8 (98)

P0309

No. 9 (68)

P0310

No. 10 (42)

P0300

⁽¹⁾

⁽¹⁾	Multiple cylinder misfire or defective Crankshaft Position (CKP) sensor.

2) Check HO2S Monitor DTCs

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

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. If none of these DTCs are present in step 1) , go to next step.

4) Check For Other DTCs

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

5) Check Rear HO2S Wiring Harness

Turn ignition off. Ensure HO2S wiring harness is correctly routed and connectors are tight. Repair as necessary. Disconnect PCM 104-pin connector. Inspect connector for loose, damaged or corroded terminals. Also check PCM pins. If PCM pins are damaged, replace PCM. If wiring harness and connectors are okay, go to next step.

6) Check Fuel Pressure

Turn ignition off. Release fuel pressure. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. On models equipped with a fuel pressure regulator on fuel rail, check vacuum hose and vacuum supply to regulator. Repair as necessary. On all models, install fuel pressure gauge. Start engine and allow to idle. Note fuel pressure gauge reading. Increase engine speed to 2500 RPM and maintain for one minute. If fuel pressure is as specified, go to next step. See FUEL PRESSURE SPECIFICATIONS . If fuel pressure is not as specified, go to TEST HC .

7) Check For Exhaust System Leaks

If exhaust system leaks, it may cause catalyst monitor efficiency test to fail. Inspect exhaust system for cracks, loose connections or punctures. Repair as necessary. If exhaust system is okay, go to next step.

8) Check For Exhaust System Restrictions

Inspect exhaust system for collapsed areas, dents or excessive bends. Repair or replace as necessary. If exhaust system is okay, go to next step.

9) Check Manifold Vacuum

Install tachometer. Connect vacuum gauge to intake manifold vacuum source. Start engine and raise engine speed to 2000 RPM. Manifold vacuum should increase to more than 16 in. Hg. If manifold vacuum is okay, go to next step. If manifold vacuum is low, go to step 11.

10) Check Manifold Vacuum For Exhaust Restriction

Leave tachometer and vacuum gauge connected. Start engine and raise engine speed to 2000 RPM. On a non-restricted system, manifold vacuum increases quickly to normal range as increased RPM is maintained. On a restricted system, manifold vacuum increases slowly to normal range as increased RPM is maintained. If manifold vacuum is okay, no indication of exhaust leak or restriction has been detected and testing is complete. If manifold vacuum is low or slow to respond, go to next step.

11) Check Manifold Vacuum

Leave tachometer and vacuum gauge connected. Disconnect exhaust pipe from exhaust manifold. Start engine and raise engine speed to 2000 RPM. If manifold vacuum is now okay, 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, restriction exists in exhaust manifold. Repair as necessary.

12) Check Manifold Vacuum

Leave tachometer and vacuum gauge connected. Disconnect muffler/tailpipe assembly from rear of catalytic converter. Start engine and raise engine speed to 2000 RPM. If manifold vacuum is now okay, fault is in muffler/tailpipe assembly. Repair as necessary. If manifold vacuum is still not okay, fault is in catalytic converter. Replace converter. After replacing converter, check tailpipe/muffler assembly for debris from faulty catalytic converter.