Disconnect O2 sensor.

Jumper harness CKT 412 to ground.

Scantool should display O2 voltage below 0.2 volts (200mV) with engine running.

Does it?

NO YES


Faulty O2 sensor connection or sensor.


Ignition on, engine off.

Check voltage of CKT 412 at O2 sensor harness connector using a digital voltmeter.

It should be 0.3-0.6V (300-600mV).


Faulty ECM.


Open CKT 413, faulty connection or faulty ECM.


Open CKT 412, faulty ECM connection or faulty ECM.


Disconnect sensor.

Jumper harness terminals together.

Scantool should display 130 C or more.

Does it?

YES NO


Faulty connection or sensor.


Jumper CKT 410 to ground.

Scantool should display over 130 C.

Does it?

YES NO


Open sensor ground circuit, faulty connection or faulty ECM.


Open CKT 410, faulty connection at ECM or faulty ECM.


Disconnect sensor.

Scantool should display TPS below 0.2 volts (200 mV).

Does it?

NO YES


CKT 417 is shorted to voltage or faulty ECM.


Probe sensor ground circuit with a test light connected to 12 volts.

LIGHT ON LIGHT OFF


Faulty connection or sensor.


Open sensor ground circuit or faulty ECM.


Disconnect sensor.

Jumper circuits 416 & 417 together.

Scantool should display TPS over 4.0 volts.

Does it?

YES NO


Replace sensor


Probe circuit 417 with a test light connected to 12 volts.

Scantool should display TPS over 4.0 V.

Does it?

YES NO


Circuit 416 open, shorted to ground, faulty connection or faulty ECM.


Circuit 417 open,shorted to ground, faulty connection or faulty ECM.


Disconnect sensor.

Jumper harness terminals together.

Scantool should display temperature over 130 C.

Does it?

YES NO


Faulty connection or sensor.


Jumper CKT 472 to ground.

Scantool should display temperature over 130 C.

Does it?

YES NO


Open sensor ground circuit, faulty connection or faulty ECM.


Open CKT 472, faulty connection or faulty ECM.


Ignition off.

Disconnect VSS buffer.

Ignition on.

Probe I.P. connector terminal with voltmeter to ground.

Voltmeter should display 10 volts or more.

Does it?

YES NO


Circuit 437 open between I.P. connector and VSS buffer or faulty buffer.


Circuit 437 shorted to ground, open between ECM and I.P. connector, or faulty ECM.


Disconnect sensor.

Scantool should display temperature below -30 C.

Does it?

YES NO


Faulty sensor.


CKT 472 shorted to ground or faulty ECM.


Disconnect EGR solenoid electrical connector.

Repeat test.

Valve moves Valve does not move


Faulty EGR solenoid.


CKT 901 shorted to ground or faulty ECM.


Ground diagnostic test terminal.

Repeat test.

Valve moves Valve does not move


Disconnect EGR 4-way connector.

Connect voltmeter between C & D.

Ignition on.

Over 10 volts Under 10 volts


Connect ohmmeter across terminals C & D of solenoid.

Apply 24 kPa (7") vacuum to vacuum diagnostic switch.

Note resistance.

Low resistance (switch closed)

High resistance (switch open)


Faulty switch, wiring or connections.


Connect voltmeter between terminal C and chassis ground.

Over 10 volts

Under 10 volts


Open ground CKT 450.


Open CKT 997, faulty connection or faulty ECM.


Disconnect EGR 4-way connector.

Connect test light between harness terminals A & B.

Light "on" Light "off"


Connect vacuum pump to EGR valve.

Apply vacuum and observe valve.

Valve moves Valve does not move


Faulty hose or connection between EGR solenoid and EGR valve or faulty EGR solenoid.


Replace EGR valve.


Connect test light between terminal A and ground.

Light "off" Light "on"


Repair open CKT 639


Open CKT 901, faulty connection or faulty ECM.


Ignition Off.

Disconnect sensor electrical connector.

Ignition On.

MAP voltage should read 1 volt or less.

Does it?

YES NO


CKT 432 shorted to voltage, shorted to CKT 416, or faulty ECM.


Probe CKT 469 with a test light to 12 volts.

Test light should light.

Does it?

YES NO


Plugged or leaking sensor vacuum hose or faulty MAP sensor.


CKT 469 open.


Ignition Off.

Disconnect sensor electrical connector.

Jumper harness terminals "B" to "C".

Ignition On.

MAP voltage should read over 4 volts.

Does it?

YES NO


Faulty connection or sensor.


Ignition Off.

Remove jumper wire.

Probe terminal "B" (CKT 432) with a light to 12 volts.

Ignition On.

MAP voltage should read over 4 volts.

Does it?

YES NO


CKT 416 shorted to ground or open or faulty ECM.


CKT 432 shorted to ground or open or faulty ECM.


Idle engine for 1 minute and note RPM.

Will not return to idle RPM recorded above

Returns to idle RPM recorded above


Idle Air Control OK.

See Diagnostic Aids for further help.


Turn ignition off.

Disconnect IAC valve harness.

Turn ignition on, engine stopped.

Ground diagnostic test terminal.

Connect a test light between each IAC harness connector terminal and ground.

No Lights, one or more circuits

Steady light or flashing on all circuits


Faulty IAC connection or valve.


Check for open or short to ground in circuit with no light.

If all circuits ok, check resistance across IAC coils. There should be more than 20 Ohms between IAC terminals "A" to "B" and "C" to "D".

OK Not OK


Faulty ECM connection or ECM.


Replace IAC Valve and ECM.


Ignition off.

Disconnect ECM connectors.

Ignition on.

Ohmmeter selector switch in the 1000 to 2000 Ohms range.

Probe ECM harness connector CKT 423 with an Ohmmeter to ground.

It should read less than 500 Ohms.

Does it?

YES NO


Probe ECM harness connector CKT 424 with a test light to 12 Volts and note light

Light OFF Light ON


Open CKT 423, faulty connection or faulty ignition module.


Disconnect ignition module 4-way connector.

Light OFF Light ON


Faulty Ignition Module.


CKT 424 shorted to ground.


With Ohmmeter still connected to ECM harness CKT 423 and ground. Again probe ECM harness CKT 424 with the test light connected to 12 Volts. (As test light contacts CKT 424, resistance should switch from under 500 to over 5000 Ohms).

Does it?

NO YES


Disconnect Distributor 4-way connector. Note Ohmmeter that is still connected to CKT 423 and ground. Resistance should have gone high (open circuit).

Does it?

NO YES


CKT 424 open, faulty connections or faulty ignition module.


CKT 423 shorted to ground.


Reconnect ECM and idle engine for one minute or until Code 42 sets.

Does Code set?

NO YES


Faulty ECM.


Disconnect O2 sensor.

With engine idling scantool should display O2 voltage between 0.35 volts and 0.55 volts (350mV and 550mV).

Does it?

YES NO


Refer to Diagnostic Aids.


CKT 412 shorted to ground or faulty ECM.


Disconnect O2 sensor.

Jumper harness CKT 412 to ground.

Scantool should display O2 voltage below 0.35 volts (350mV).

Does it?

YES NO


Refer to Diagnostic Aids.


Faulty ECM.