I've been researching using pre and post cat O2 waveforms to diagnose misfires and ran across this quote from the article, Intermittent Engine Misfire Analysis:

Keep in mind that the downstream, post-catalyst oxygen sensor can often be used to differentiate between an ignition- or fuel-related misfire. To illustrate, a misfire caused by a lean A/F mixture will drive the downstream voltage lower because an excess of oxygen is passing through the catalytic converter. In contrast, an ignition misfire will drive the downstream O2 voltage higher because raw fuel and oxygen is being oxidized in the catalytic ­converter.

I'm having a little trouble understanding exactly what's going on in the case of an ignition misfire. In that case you also have excess oxygen passing both sensors I would think. I don't quite understand how the operation of the cat in each case affects how the post cat waveform looks.

  • 1
    unburnt fuel would show rich in the 02 sensors. you could just the fuel trim in the freeze frame to get an idea of where you should go next. then go on to check relative compression, ignition etc... also your link points to a video about wheel bearings?
    – Ben
    Dec 14, 2017 at 0:01
  • @Ben - I think I have the right link for what Robert was trying to post. Dec 14, 2017 at 0:27

5 Answers 5


A cat converter is designed to store excess oxygen and use it to burn off excess fuel, oil, coolant that comes into the cat. If there's too much fuel coming into the cat, you wind up with a run away condition where the temp gets so hot that it melts the ceramic brick, totally clogging the exhaust and rendering the cat useless. So the article is telling you to use the post O2 sensor reading to determine if the misfire is due to a lack of fuel (clogged fuel injector, vacuum leak, etc), or too much fuel (spark plug didn't fire so none of the fuel was ignited and is now being burned in the cat instead of the cylinder. So you really DO care whether it's fuel or spark because a spark misfire will destroy the cat, costing you about $1,000. If it's lack of fuel, the cat will just see excess oxygen, which won't destroy it


A misfire in which the fuel injector injects a mixture that is too lean to ignite will result in both oxygen sensors seeing a lean mixture. This will result in the sensor output of little or no voltage. Usually 0.2 volts or less.

A fully fueled misfire such as is seen when fuel is injected but spark fails will result in a fuel air mixture exiting the cylinder and thus being burned in the catalyst. In this case both oxygen sensors will see a stoichiometric mixture. Sensor output voltage is different in this case. The pre-catalyst sensor may not read accurately due to fuel saturation. A ossilliscope reading of this sensor with a fueled misfire will show a non-standard, disorganized line with much hash seen. The post-catalyst sensor does not suffer this problem because most of the excess fuel and oxygen is consumed in the catalyst. It will read an accurate mixture, usually, but not necessarily, a voltage of 0.5 volts or higher.

A standard oxygen sensor measures neither oxygen nor fuel content in exhaust gasses. It can only indicate whether the mixture is rich or lean. It does this by burning all the fuel present in the exhaust. It outputs a voltage only when oxygen is needed to support combustion and the sensor surface.


In the second case the available fuel is being burnt in the cat - which is what they state, in the first case there is not the fuel to do that... Also post catalyst oxygen sensor is after the cat not before...

  • why does it matter to the post cat O2 signal if the fuel and air are burnt in the cylinder or in the cat? Dec 13, 2017 at 22:00
  • @RobertS.Barnes - For most all (except fairly new) rear O2 sensors it doesn't matter to them. The idea is to use it as a diagnostic tool. Dec 14, 2017 at 0:10
  • well maybe they don't like drinking raw fuel ha ha ha Feb 13, 2022 at 17:19
  • Misfire due to no fuel will always read lean, because there is no fuel present to consume the oxygen
  • Misfire due to no spark ignition:
    • Will read lean before the cat as there will be lots of air (oxygen) passing the sensor (as well as unburnt fuel).
    • Will read close to lambda 1 after the catalyst due to the catalyst encouraging the unburnt fuel and air to react, much like they would have done in the cylinder during normal non misfire combustion.

Hi I recently noticed that the downstream on my dodge ram is going low, but only when accelerating to 2000 rpm? What is the cause?

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