My truck is reporting a code P0420, which apparently is "Catalyst System Efficiency Below Threshold (Bank 1)" which can either be the catalytic converter or the rear O2 sensor. The code came on at about 30k miles, so I doubt it's the converter. So my question is, is there a way I can test the O2 sensor without a "scan" tool or oscilloscope? I'd rather not have to buy a new O2 sensor just to see if the old one is not working.

This is on a 5.7L Hemi V8.

3 Answers 3


I've diagnosed this condition many times with a vehicle-specific scan tool, and I don't think it can be done without this or an oscilloscope. O2 sensors work in the range of less than 1 volt (.2v to .7v) In particular, the rear O2 sensor should stay at a fairly constant .5v if the cat is working right. Wild swings to the extremes indicate the cat isn't working.

The voltage fluctuates rapidly and is so small that a normal DVOM won't work well (it will average the readings). I suppose a analog meter with a small enough scale may work, but you'd have to pierce the insulation, as all of the wires have to stay plugged in for reference voltage purposes.

It's also possible that it's an intermittent problem, in which case historical data and real-time data while driving the car becomes important.

A gas analyzer would tell you about the efficiency of the cat, but that's even more of a hassle than a scan tool..

Edit: If you have two rear O2 sensors, and they have the same connector and wire lengths, you can swap them. Then reset the engine codes (disconnect battery) and drive it until the light comes back on. See if the error code went to a different bank. If so, it is unlikely that you have two faulty sensors, and the problem is the cat or something else up the line from the sensor.

You can also check the physical condition of the sensor and make sure there isn't corrosion/water/oil in the connector.

  • 1
    You can tell the basic condition without an o-scope. If you use a digital VOM and just read the o2 sensor voltage, once you're operating at NOT and stoich conditions the sensor should "switch" from high to low voltage (upstream sensor) and the rear sensor should be fairly constant.
    – kkeilman
    Mar 9, 2011 at 22:59
  • It may be possible. My concerns are (1) the DVOM will average the readings, and the sensor fluctuates pretty rapidly. Setting the DVOM manually to the lowest range should help this. (2) You'd have to leave everything plugged in, which means piercing the wire insulation or get a very thin lead to slip next to the wire into the sensor in order to get a reading. A lot of this depends on the sensor design. You have to be very careful not to damage anything while testing, or risk weird issues in the future as the wire starts corroding.
    – S_Niles
    Mar 9, 2011 at 23:35

Also if the converter is actually degraded, the downstream o2 sensor output will start to look more like the upstream one - it will switch from high to low voltage on a regular basis instead of staying more steady at a constant voltage.


Easy swap the bank 1 sensor with bank 2 see if the fault follow the sensor Also make sure your exhaust manifold isn't cracked or the gasket isn't bad. But swap the sensors first.

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