DTCs on Protege

I did have a plug wire that wasn’t connected to the dist. cap and it was misfiring but that was years ago. It seems if that would have been the cause the code would have been sent a lot sooner. But don’t know. I guess there’s no easy way to test a cat just have to take it off and inspect it?

I think a properly running CAT should make the rear O2 sensor run at a steady voltage around .5 volts.


Don’t worry too much about the resistance of the sensor

The signal voltage is the main thing

It sounds like the cat isn’t doing much of anything

The rear sensor signal voltage should be very steady, NOT fluctuating like the front

It’s quite possible that the previous misfire did in the cat

You could measure backpressure, but it really doesn’t sound good for the cat at this point

Is there still a slight chance that the rear sensor could be damaged and not the CAT?


If you want to save money, replace front AND rear sensors, then clear the code.

All the monitors will reset.

Check the car after several days of normal driving.

If the cat monitor runs to completion without setting the same code, you’re good.

If the same code is back (or pending), the cat may very well be a goner.

Are you sure you don’t have any exhaust leaks?

Speaking of your previous misfire, I’d make sure ALL of the plugs, wires, cap, etc. are new, or at least you know they’re good.

I’d hate for you to replace a cat, only to have it ruined by bad plugs, wires, etc.

Are you fairly certain you don’t still have any misfires?

Bad/plugged injectors can certainly cause a misfire.

Bottom line: you’re going to have to spend some bucks. One way or the other.

Going by the voltage readings you showed from the two sensors it looks like the rear sensor is basically following the front sensor readings meaning the CAT is basically doing nothing. The O2 sensors seem to be working fine from what I can tell.

@mystic … The front sensor seems from what you report like it is working. Here’s the basic chemical reaction going on during combustion in the cylinder.

2 CH2 +3 O2 = 2 CO2 + 2 H20

The CH2 comes from the gasoline, the O2 from the air. For a proper mixture, you need exactly 2 CH2’s to combine with 3 O2’s. If there’s too much CH2, you burn all the O2, but you get some CH2 (hydrocarbons) coming out the tailpipe, which is no good. If there’s not enough CH2, not all the O2 is burned, and you get O2 out the tailpipe, which is also not good.

So what the ECM (the engine computer) does is play a sort of cat and mouse game. It monitors the amount of O2 in the exhaust (the front O2 sensor) with a detector very sensitive to O2. Ideally there would be no O2 in the exhaust, and there wouldn’t be any CH2’s either. Only CO2’s and H20’s. But there are no CH2 sensors available to the ECM, so it has no direct way to tell about the CH2 situation. So what it does, it has to work with what’s it’s got to work with.

If the ECM senses O2 then, it juices the gas a tad so there is more of the CH2’s to combine with the spurious O2’s. When the ECM doesn’t sense O2, never willing to leave good enough alone, it worries there are CH2’s now coming out the tailpipe. So it cuts back on the gas, until O2 re-appears. etc, etc.

So when everything is working correctly, what you are seeing is exactly what you should be seeing, at least in terms of the front O2 sensor. The voltages you report are typical of the response of a Zirconia narrow band O2 sensor. What isn’t entirely clear is how rapidly your O2 sensor is responding. It should respond very quickly to O2 changes. Mechanics measure this by how fast the O2 signal oscillates from one end of the scale to the other, too much O2, to not enough O2, as the ECM juices the gas, then cuts it back again.

I’m assuming that I don’t have any misfires because no misfire codes have come up. Is it possible to have misfires without the computer throwing a code? I’ll check the fuel injectors but everything else in terms of plugs, wires, cap, & rotor have less than 20k miles on them. I can still check to see that all connections are solid.

I’m still going to check for exhaust leaks and I picked up a fuel pressure tester today - is it still worth it to check the fuel pressure?


Here’s a simple question

Is the engine at least running very smoothly?

At all times?

Every engine misfires to a degree

When a certain threshold is exceeded, a code will be generated

As far as the exhaust leaks go, here’s what I recommend. This is not very scientific, but it’s simple, and I actually use this method in the shop from time to time.

Put the vehicle on 4 jackstands
Start the engine
Slide under the vehicle, on a creeper
Carefully put the palm of your hand next to all the places on the exhaust where a leak is most likely to occur . . . exhaust manifold, flange, gaskets, etc.
If there is a significant leak, you’ll more than likely feel it. Sometimes you’ll even hear it.
Don’t actually touch anything!

The engine does run very smoothly as far as I can tell. The only thing that would lead me to believe that it doesn’t run well is when I have the scan tool hooked up and watch as the idle jumps around from 816, 831, 822, 808, 833 and so on - but this, I’m guessing is either normal or a slight vacuum leak somewhere. (vacuum gauge is on my to get list)


Have you cleaned the throttle body yet?

Nope. You think that’s why it idles the way it does?

I was going to let this thread RIP but then I found this:

The most important thing to remember about Oxygen Sensors is that they were never meant to last the life of the car! They are wear items and need to be replaced as you would brakes, tires or spark plugs. When Oxygen Sensors reach 80,000 miles plus…change them. Catalytic Converters do not go bad by themselves, 95% of the time some other issue with the vehicle will destroy the cat and most of the time it is the Oxygen Sensors. Even when you have them checked and the tech says they are operating okay…if the mileage is over 80,000 change them. When sensors reach that age they may still be functioning but the signal they send becomes lazy and since they operate in millivolts the slightest signal fluctuation can cause an air/fuel mixture problem and that in turn will take out the cat. If you are in the market for a converter and the vehicle still has the original Oxygen Sensors you should strongly consider their replacement…if not you could be buying another converter 90 days down the road.



While oxygen sensors don’t last forever, I would be very hesitant to say they are the cause for a cat’s demise

For the most part . . . bogus

Why are there lots of cars on the road with hundreds of thousands of miles on them that still have the original cat and oxygen sensors, and they’re still running well and passing the smog test?

Pretty much what I figured…just a company trying to scare people into buying parts they don’t need.


You wouldn’t replace your engine just because it had miles on it, would you?

nope : ) well…maybe if the miles we’re talking here were somewhere in the 2,000,000 + range. ; )