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How to diagnose a clogged (plugged) catalytic converter

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BEFORE YOU START
  • Do not carry out this intervention on a hot engine, Wait at least 90 minutes after turning off the engine.
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Our pictorials are of informative nature and do illustrate works carried out by ourselves at our own premises for our own use and satisfaction.
Readers who envisage to carry out similar works inspired by the below method shall do this at their own risks and liability.

Symptoms of a clogged (plugged) catalytic converter

  1. Noticeable loss of power when accelerating upwards in slopes
  2. Engine refuses to rev up beyond 3,000 rpm under load
  3. Fuel consumption increases by 1.5 l per 100 Km.
  4. Engine maintains a good torque when driving slowly even when climbing (at the early stages of the problem)
  5. Triggers all possible turbo low pressure alerts and turns on the check engine light: "turbo circuit pressure too low", "turbo regulation valve suspected jammed open", "compressor discharge valve suspected jammed open".
  6. ECU forces the turbo waste gate valve to remain open upon a strong acceleration attempt (if this happens, turn off and on the engine to reset the ECU then drive slowly till your destination)
  7. Downstream sensor still responds normally since the catalytic material itself is not damaged. The flow is reduced due to the clog but the cat still converts the toxic gases into CO2 and H20 which prompts the downstream sensor to return correct values.

Causes of a clogged (plugged) catalytic converter

It's almost never a single cause behind a catalytic converter clogging.
  • No car is perfect so oil consumption, less performant sensors, etc. reduce its lifetime.
  • No fuel is perfect, additives can lead to a slow build up of various coatings on the catalyst material
  • No roads are perfect, traffic jams in particular do not allow engines to breathe normally, while the combustion is not at its best and the catalytic converter is relatively cold....
The below pictures show a brown colored coating covering the surface of the catalytic converter's honeycomb.
It could be a manganese oxide compound that formed with time until constituting this sticky cat clog.
This catalytic converter had 112,000 Km of lifetime:
A normal catalytic converter monolith allows the light to pass through:

How to diagnose a plugged catalytic converter 

WITH THE CONVERTER STILL ON CAR:
Using an inspection camera (endoscope) while the catalytic converter is still on the vehicle.
Remove the upstream O2 sensor and check if you see the small squares of the ceramic monolith all clear and black.
Or if you see a white/yellow compound covering the squares like in this case:
- A few squares are visible to the upper left corner
- All others are clogged with a white compound
Remove the downstream O2 sensor and introduce the inspection camera.
You should see the difference as all squares are clean and well visible.
AFTER CONVERTER REMOVAL:
Check for the presence of debris or a fine powder denoting the ageing of the ceramic monolith and its gradual deterioration.
You need then to replace your catalytic converter.
If you decide to suppress the catalyst material, you need to know that:
  1. This is illegal
  2. The check engine light will remain ON
You can however proceed with the catalytic converter gutting as a temporary fix in case a replacement is not immediately available in the market. If you do so:
  • Do not cut the catalytic converter's body.
  • Introduce any rigid steel rod like an 8 mm threaded rod, an old screwdriver, etc. which you will bend differently each time and brake the ceramic honeycomb piece by piece.
  • Make sure to remove the ceramic fiber blanket (right) used all around the monolith to protect it from shocks. It can obstruct the exhaust if you leave a piece in there.
  • REMEMBER THIS IS ILLEGAL

See also...

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