On my 2005 dodge ram Daytona I have two codes, P0133 and P0420. Can a low cat efficiency be due to a slow responding o2 sensor?

  • Has the O2 sensor ever been replaced? Has any exhaust work been done to the car recently?
    – Zaid
    Aug 19 '15 at 2:58
  • No but I'm going to replace it this weekend. No exhaust work has been done.
    – Sumner
    Aug 21 '15 at 23:20

Yes, the P0420 catalytic efficiency can be caused by a bad O2 sensor.

But a P0133 refers to the front O2 sensor so things are a little more complicated than when people refer to a bad rear O2 sensor.

Your front O2 sensor is probably bad (P0133) and your cat and/or your rear O2 sensor may be bad (P0420).

One cheap test for the cat is using a laser infrared thermometer to measure the before cat and after cat temps. Point it at the pipe right before the inlet to the cat and then the pipe right after the cat. I don't know exactly what your numbers should be, but you should see a difference on the order of 100 degrees fahrenheit.

If you do see a temp change, your rear O2 sensor is probably bad. If there is no temp change, there is a chance the rear O2 may be bad, but maybe not.

N.B. for others: cat refers to the catalytic converter; front means before cat; rear means after cat

Also check out this related question.


Yes. The rear sensor is used to calculate the difference in exhaust gas quality before and after the cat. In the simplest terms the ECM compares the voltages of the front and rear O2 sensors and if they're too similar, you get the P0420 code. Now, because the front sensor is giving faulty data to the ECM (P0133), the formula the ECM uses to calculate cat efficiency will yield inaccurate results. In this case, because the front O2 sensor sends garbage and the rear may be sending accurate data, it looks like the cat isn't working properly.

It's basically a case of the front O2 sensor lying to the ECM and causing (potentially) phantom errors further along.

The usual fix for a P0133 is to replace the sensor and inspect the wiring.

  • OK thanks for the info. Front o2 sensor will be replaced this weekend and we'll see what happens.
    – Sumner
    Aug 21 '15 at 23:22

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