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I am having a problem with a 1998 Malibu. I keep getting several codes but primarily P0420. I have replace the Catalytic Converter twice. Both oxygen sensors have been replaced. I have checked the Mass Air Flow and EGR and they both appear to be functioning correctly. Can anyone provide ideas to resolving this? Below are the output from Carista and Torque.

Thanks

Carista:

OBD2 - confirmed:
P0102 - Mass or Volume Air Flow "A" Circuit Low
P0122 - Throttle/Pedal Position Sensor/Switch "A" Circuit Low
P0401 - Exhaust Gas Recirculation "A" Flow Insufficient Detected
P0420 - Catalyst System Efficiency Below Threshold Bank 1

OBD2 - pending:
P0401 - Exhaust Gas Recirculation "A" Flow Insufficient Detected
P0420 - Catalyst System Efficiency Below Threshold Bank 1
P1441

Torque:

Fault log report generated by Torque for Android
=================================================

Vehicle VIN: Not present
Vehicle Manufacturer: Unknown
Vehicle Calibration ID: Not present

Current Fault Log
------------------
P0102: Mass or Volume Air Flow Circuit Low Input
P0122: Throttle/Pedal Position Sensor/Switch A Circuit Low Input
P0401: Exhaust Gas Recirculation Flow Insufficient Detected
P0420: Catalyst System Efficiency Below Threshold (Bank 1)

Pending Fault Log
------------------
P1441: [BMW] Leakage Diagnostic Pump Control Open Circuit
[Chrysler] Evaporative Emission (EVAP) System Flow During Non-Purge
[Nissan] Vacuum Cut Valve Bypass Valve
[Dodge] Evaporative Emission (EVAP) System Flow During Non-Purge
[Jeep] Evaporative Emission (EVAP) System Flow During Non-Purge
[Seat/Audi/Volkswagen] EGR Valve Circ Open/Short to Ground

Torque:

Mode $06 report generated by Torque for Android
================================================

Vehicle VIN: Not present
Vehicle Manufacturer: Unknown
Vehicle Calibration ID: Not present

Unit and scaling information are not supplied with the data from the ECU for this type
of vehicle. Consulting the manufacturers service book for this information is recommended.

Test report:
------------------
TID:$02 CID:$04
- Lean to Rich sensor threshold voltage(constant)
Test incomplete or dependant test failed
--
TID:$02 CID:$10
- Lean to Rich sensor threshold voltage(constant)
Min: 79
Test result value: 79
PASS
----
TID:$02 CID:$11
- Lean to Rich sensor threshold voltage(constant)
Min: 250
Test result value: 171
FAIL
----
TID:$02 CID:$20
- Lean to Rich sensor threshold voltage(constant)
Max: 250
Test result value: 15
PASS
----
TID:$02 CID:$26
- Lean to Rich sensor threshold voltage(constant)
Max: 100
Test result value: 8
PASS
----
TID:$02 CID:$30
- Lean to Rich sensor threshold voltage(constant)
Test incomplete or dependant test failed
--
TID:$02 CID:$31
- Lean to Rich sensor threshold voltage(constant)
Max: 110
Test result value: 138
FAIL
----
TID:$02 CID:$40
- Lean to Rich sensor threshold voltage(constant)
Test incomplete or dependant test failed
--
TID:$02 CID:$46
- Lean to Rich sensor threshold voltage(constant)
Min: 65
Test result value: 65
PASS
----
TID:$06 CID:$35
- Lean to Rich sensor switch time(calculated)
Max: 111
Test result value: 47
PASS
----
TID:$06 CID:$41
- Lean to Rich sensor switch time(calculated)
Max: 342
Test result value: 49
PASS
----
TID:$07 CID:$0c
- Minimum sensor Voltage for test cycle(calculated)
Max: 0
Test result value: 65,491
FAIL
----
TID:$07 CID:$0d
- Minimum sensor Voltage for test cycle(calculated)
Max: 1
Test result value: 2
FAIL
----
TID:$0c CID:$20
- Misfire counts for last/current driving cycles(calculated)
Max: 0
Test result value: 33,536
FAIL
----
TID:$02 CID:$04
- Lean to Rich sensor threshold voltage(constant)
Test incomplete or dependant test failed
--
TID:$02 CID:$10
- Lean to Rich sensor threshold voltage(constant)
Min: 79
Test result value: 79
PASS
----
TID:$02 CID:$11
- Lean to Rich sensor threshold voltage(constant)
Min: 250
Test result value: 171
FAIL
----
TID:$02 CID:$20
- Lean to Rich sensor threshold voltage(constant)
Max: 250
Test result value: 15
PASS
----
TID:$02 CID:$26
- Lean to Rich sensor threshold voltage(constant)
Max: 100
Test result value: 8
PASS
----
TID:$02 CID:$30
- Lean to Rich sensor threshold voltage(constant)
Test incomplete or dependant test failed
--
TID:$02 CID:$31
- Lean to Rich sensor threshold voltage(constant)
Max: 110
Test result value: 138
FAIL
----
TID:$02 CID:$40
- Lean to Rich sensor threshold voltage(constant)
Test incomplete or dependant test failed
--
TID:$02 CID:$46
- Lean to Rich sensor threshold voltage(constant)
Min: 65
Test result value: 65
PASS
----
TID:$06 CID:$35
- Lean to Rich sensor switch time(calculated)
Max: 111
Test result value: 47
PASS
----
TID:$06 CID:$41
- Lean to Rich sensor switch time(calculated)
Max: 342
Test result value: 49
PASS
----
TID:$07 CID:$0c
- Minimum sensor Voltage for test cycle(calculated)
Max: 0
Test result value: 65,491
FAIL
----
TID:$07 CID:$0d
- Minimum sensor Voltage for test cycle(calculated)
Max: 1
Test result value: 2
FAIL
----
TID:$0c CID:$20
- Misfire counts for last/current driving cycles(calculated)
Max: 0
Test result value: 33,536
FAIL
----

End of report.   
3

The first thing to do is too take the EGR out of circuit. A quick and easy way to do this is to cut an old metal oil can(its thin enough for scissors) so you have a piece of the can that will fit between the bolts of the EGR. Tighten the EGR bolts up with the can acting as a gasket. Now try watching the MAF sensor voltage as you rev the engine sharply and off again as soon as you get the revs up. A good MAF voltage will rise to around 4 volts. The voltage rise to around 4 volts will prove the MAK is Ok. You can try the MAF voltage as a before and after test of the EGR being blanked off. If you get an improvement you know its the EGR at fault. Looking at your description of the fault you have to be critical of the EGR as a faulty EGR will give you a host of 'ghost' codes which seems to be the case.

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