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I have an 04 TSX that was showing a P0420 - Catalyst System Efficiency Below Threshold Bank 1 and the check engine light was on. I tried to pass smog and it failed of course. I changed the cat converter and the downstream sensor and the check engine light is still on. Any ideas what the problem is. Should I change the upstream sensor as well? Someone told me just use the engine cleaner fluid in gas tank, would this work? Thanks

2005 Acura TSX
3 Posts
Hello, hope you are well aside from dealing with this!

I have a few thoughts!

If the ECU was not reset fully after the O2 sensor and cat replacement, it will hang on to stored values and could be contributing to your issue.

There are two ways I like to reset an ECU: quick and dirty, or an overnight sit.

Quick and Dirty:
  1. Disconnect the positive terminal from the battery
  2. Turn the ignition on to 'II' (just before cranking)
  3. Turn the headlights 'on'
  4. Grab a book or a phone to entertain yourself
  5. Sit in the car....
  6. Depress the brake pedal, again with the car in the 'running' ignition key position
    1. Keep it depressed for at least 5 to 10 minutes.
  7. Reconnect the battery, quite obviously
  8. Go drive your car on the interstate for at least 5 minutes
Long but Sure Fire:
  1. Disconnect the positive terminal from the battery
  2. Let it sit at least 12 hours
  3. Reconnect battery
  4. Done
By replacing only the downstream, your car was left with an old, less efficient sensor upstream.

The O2 sensors work along with the ECU, put plainly, to determine air to fuel ratios based on a few different factors, and the two sensors compare their readings against one another for both diagnostic, vehicle operation, and emissions purposes.

If the upstream sensor is faulty it could be causing your vehicle to run rich, thereby slowly fouling the catalytic converter and likely causing the downstream sensor to develop a patina on the sensor surface (reduces accuracy of readings) due to the excess of fuel in the exhaust. This can foul your plugs eventually, as well.

If you don't want to replace the upstream sensor right off the bat, I highly recommend a product called CataClean. It is made for removing carbon from catalytic converters, cylinders, and intake manifold, and you guessed it..O2 sensors. It's about $8 for a bottle. Empty it in to the gas tank with about 1/4th a tank of gas in the vehicle, drive it 10-20 miles, fill the car up with 93 octane and go for a short joy drive - 30 miles or so, interstate speeds.

And worst, LEAST LIKELY scenario..the ECU could be faulty. Don't think about that past being aware it's a very faint possibility.

No guarantees this information will help, but after owning and doing all the work on 31 different vehicles for two decades this Saturday, the logic follows in my mind. I work with small electronics as well, and it is easy to underestimate just how sensitive the electronic devices used to capture data are to voltage fluctuations.'s $8 or so for a bottle of the stuff, versus much more for another O2 sensor. Fairly logical gamble.

Hope this helps!!
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