Catalytic Converter: Testing and Inspection

Three Way Catalytic Converter (TWC), checking

Special tools and equipment






- VAG1551 Scan Tool (ST)

Note: If VAG1551 Scan Tool (ST) is not available, use VAG1552 Mobile Scan Tool (ST) instead, although this tool cannot produce a print-out.






- VAG1 551/3B cable

Test requirements





- All fuses must be OK.
- Exhaust system must be properly sealed between catalytic converter and cylinder head.
- Parking brake must be engaged or else daylight driving lights will be switched on.
- Coolant temperature must be at least 80 °C, = Display group 04, display field 3
- Catalytic converter temperature must be at least 350 °C = Display group 46, display field 2
- All electrical consumers like, for example, lights and rear window heating must be switched off.
- If vehicle is equipped with an A/C system, it must be switched off.

Note: This diagnostic can only be completed if the diagnostic of oxygen sensor aging has already been performed successfully.

Test sequence
- Connect VAG 1551 scan tool (VAG1552). Start engine and select the Engine Control Module (ECM) using address word 01. Connecting scan tool and selecting Engine Control Module (ECM).
- Check DTC memory, repair malfunctions if necessary and then erase DTC memory.
- Then:
- Connect Scan Tool (ST) VAG1551





Indicated on display:
- Press buttons -0- and -4- to select "Basic Setting" function 04. Press -Q- button to confirm input.





Indicated on display:
- Press buttons -0-, -4- and -6- to select "Display group number 46". Press -Q- button to confirm input.





Indicated on display: (1 to 4 = display fields)
- Depress brake pedal and hold down.
- Depress accelerator pedal until it is in Wide Open Throttle (WOT) position.

The Engine Control Module (ECM) raises the engine speed to about 2300 RPM.
- Maintain brake pedal and accelerator pedal depressed until the display in display field 4 jumps from "Test OFF" to "Test ON". Catalytic converter temperature in display field 2 must be at least 350 °C.

Note: Diagnostic takes approximately 1 minute.

- Maintain brake pedal and accelerator pedal depressed until the result of the function test is indicated in display field 4. Specification: "Cat B1 OK"
- Release brake and accelerator pedal.
If display field 4 indicates "Cat B1 not OK":
- Check DTC memory.
- Repeat test.
If display field 4 indicates "Cat B1 not OK" again:
- Check DTC memory again.

If no other malfunctions were recognized by the Engine Control Module (ECM), catalytic converter damage could be the cause.

Note: This test is designed for vehicles with On Board Diagnostic (OBD) and mentions possible catalytic converter damage. If "Cat B1 not OK" is indicated, the catalytic converter does not generally need to be replaced, only if the following emissions test is not passed or visible damaged to the catalytic converter can be detected.

If the emissions test is not passed or damage to the catalytic converter is detected:
- Replace catalytic converter.
- Read readiness code.
- If DTC memory was erased, readiness code must be re-generated

If specified value "Cat B1 OK" is obtained:
- Press -> button.





Indicated on display:
- Press buttons -0- and -6- to select "End data transfer" function 06. Press -Q- button to confirm input.