99 Miata P0420

Ok I’m stumped. 99 Miata with a P0420 code. Front sensor and back sensor (O2) are both real active. Cat is 120 hotter in the rear than the front, so it appears to be burning. What gives? Cat going bad? Why is the rear O2 sensor so active if the cat is burning?

More on the Miata: (I can’t find the edit button). car runs fine, strong. No other codes. Goes into closed loop normally. And when I disconnect the front sensor, the rear one stops working. benny

When was the last time you changed the plugs and wires ?

BTW the pencil icon is the edit button. It will only show up in your messages.

Not sure about the plugs and wires, not my car. I would think I’d get a misfire cel if one of them were not firing. I guess you’re suggesting they might still be firing, but not at 100%? Therefore not burning at 14.7:1. That’s an idea. Any others?

Are the coolant temp sensor for the ECU and MAF sensor working as they should? If they are ok then there could be a problem inside the CAT I suppose and it is time for a new one.

A point to remember is that, unless they have changed it, the Miata fires each plug twice as often as other cars, so they wear faster.

Let’s kick this in the pants to move it up front so it doesn’t get buried deep into the files. I reviewed a wiring diagram of the O2 sensor’s and saw how they are tied together, so it does make sense that when one is disconnected, the other one will electrically fall off line. I’m still wondering why, if the cat is burning, why is the rear sensor so active, indicating presence of oxygen.

Let’s bump this to the front. Still wondering why the back O2 sensor is so active with the cat burning.

The description for the DTC P0420 code is ‘warm up catalyst system efficiency below threshold’. With a working cat there will be a delay between the inlet O2 sensor and the outlet O2 sensor voltage toggles. That delay indicates that osygen is collecting or being removed from the catalyst bed. If the two sensors are tracking each other it indicates that the catalyst is not storing oxygen. A dual trace oscilliscope is a good way to visualize this. The CC still could be burning off exhaust emmissions but not doing it efficiently.

Maybe someone else has a way to test the efficiency of the catalyst separate from the OBD system. Other than that changing the catalytic converter may be the only way to eliminate the CC as the culprit.

Hope this helps.

How many miles does the car have ? I would replace both sensors if there is more than 100k on them and check if it is not a faulty sensor, but only after I check all other possible problems, spark plugs, wires, MAF, air filter and is the exhaust system is without holes or cracks between the Ox sensors. You may need a service manual to troubleshoot, the attachment is about the Ox sensors.

I’m a bit confused by the question. If the catalytic converter is functioning properly there downstream oxygen sensor will properly sense a higher level of oxygen.

The first stage of the converter seperates the oxides of nitrogen into seperate oxygen and nitrogen atoms. Some are “grabbed” by carbon monoxide (CO) molecules in the second stage “burn” to create carbon dioxide (CO2), but CO is incomplete combustion, a sign of excess fuel-to-oxygen, low combustion temperatures, or an additional uncontrolled hydrocarbon such as oil entering the cylinders. If an engine is operating cleanly, CO levels should be low and the oxygen atoms freed from the nitrogen in the first stage of the converter should be elevating the oxygen levels sensed by the downstream oxygen sensor. And the signal from the downstream converte should be smoother than the one from the upstream converter. It shoudl not also be “real active”.

The code is telling you that the downstream converter is not sensing a sufficient change in oxygen levels from the upstream converter. Put simply, you’ve found an unacceptable signal downstream and, since the downstream converter is apparently working and the signal not far different from the upstream signal, the converter is bad. You’ve correctly diagnosed a bad converter. My guess is that the cayalyst in the forst stage of the converter has become coated with combustion byproducts and the exhaust molecules can no longer contact the honeycomb surfaces readily enough to properly cause speeration of the NOx molecules. Surface contact is necessary for the catalyst to work.

Or did I miss something?

I disagree with the rhetoric that I see given from apparently knowledgeable people.

As far as O2 content, the catalyst is like a capacitor. When free/excess O2 is present (LEAN), it should soak it up. When too little is present (RICH) is should release it for reacting with the excess hydrocarbons. The down stream sensor, within limits, should NEVER see the change. It should be relatively constant in the normal RICH:LEAN process.

As catalyst efficiency degrades, the instances of like readings increases, separated by shorter and shorter delays. Once the readings are identical, but only separated by a delay, the cat is deemed inert.

The system is tolerant of so many like states between front and rear. The closer to zero this number is, the higher the cat efficiency. The higher the number, the lower the cat efficiency.

Now there are surely instances where like states would be sensible. Like when fully warm and traveling down a long(er) grade. Depending on the fuel strategy, the injectors may be 100% off. That would cause both front and rear to read HIGH O2. I would assume that the ECM/PCM is configured to ignore these states.

I’d clean or replace both sensors in case they’re getting lazy. The overshoots on the transitions may run the fuel mix outside of the cat’s current efficiency capacity to handle it.

The catalyst doesn’t soak up oxygen. The first stage actually temporarily collects nitrogen atoms. I’ve attached a link explaining the processes involved

It isn’t a great link, but it’ll do.

The oxygen in the NOx molecules is freed to be picked up by the CO molecules to create CO2 in the “second burn”. HC molecules also “burn”, breaking apart and the carbon atoms picking up oxygen to form ideally CO2, the nitrogen atoms passing out as nitrogen or N2. Nitrogen atoms from the NOx that are temporarily attracted to the catalyst in the first stage are “washed” away by other passing nitrogen molecules.

The goal of the converter being to reduce NOx, CO, and unburned HC levels, It’s method of doing so converts the various exhaust constituants to nitrogen, oxygen, and carbon dioxide. The level of oxygen rises, since the exhaust stream now contains more free oxygen, it having been freed from the NOx.