Another P0420 story...help!

IF:
there are NO exhaust leaks (not even a pinhole) from the cylinder head to 18" past the downstream oxygen sensor; AND the engine runs normally (NO misfires plus fuel trims are 0 +/- 10% at idle and while driving); AND both the upstream air-fuel and downstream oxygen sensors can read full range rich-to-lean; (also no TSB's from the manufacturer concerning a P0420);

THEN I replace the catalytic converter to resolve a low cat efficiency code.

I have seen OE cats go 200-300k plus miles without setting codes. I have seen AfterMarket cats throw codes after a couple months. If the AM part sells for 25% or less than the OE, I can only imagine that it has much less precious metal than the OE.

That said, something is wrong with your car- I typically see the downstream O2 sensor read ABOUT 700-800 mV when the cat is hot and "lit off". 940 mV is high- esp if it stays there, that's reading fully rich. You would need to see what fuel trims are doing and figure out what's going on. Maybe that O2 sensor is lying. Maybe the upstream A-F sensor is lying. Maybe something else is wrong that's causing the engine to run rich.

I wouldn't think that sensor "spacers" would work on this car. Spacers "help" by creating a pocket of stagnant exhaust gas around the tip of the O2 sensor. This slows its response, which can "fool" some older ECMs that compared the lean-rich switching rates of the upstream vs downstream O2 in order to test the cat function.

The prius (and most newer cars) use a test called Oxygen Storage Capacity (OSC). Google it if you want to know more about it, but a spacer isn't going to affect that type of test much.

Good luck.

Posted via the PriusChat mobile app.

 

I'm not sure if you logged the upstream and downstream O2 sensors but if not, that is the best way to determine if the cat is outside it's efficiency range. Since you replaced both O2 sensors with new (and other parts) and if you have no exhaust leaks as Mr_guy_mann suggested, you will need to log the O2 data and compare them. If both signals look pretty much the same with a very slight delay for the rear (being the rear and farther down the exhaust stream) that is a pretty clear indication the cat is not doing a good enough job. I'm not sure if you're just taking quick reads of the data, like on a dashboard in Torque or if you have logs for over a period of time to evaluate after the drive. I know for me, reading the dashboard for fast signals like the O2 sensors was worthless.

If logging the data, make sure you are sampling fast enough. 500ms or 1sec intervals are good enough time intervals to show accurate data, but it's important not to use collecting data by PID frame since that will alias the fast signal data and not be very useful. Since the Prius is a strange creature turning on and off the engine often it is very helpful to also capture data for RPM and vehicle speed so you can see when the engine is off or on and when you're at a stop. Seeing the rear O2 drop to 0 on a gas engine vehicle would be alarming, but on a Prius is completely normal when the engine turns off which it does often.

There might be better, but I found OBD Fusion to be my app of choice for data logging. I just had a lot better results than something like Torque when only using a BT OBDII adapter. I don't have an expensive scantool that can log data, which would likely be even better. You can use datazap.me | Fast, Free, Interactive Datalogs on the web for quick and dirty looking at the log. That site can turn the raw .csv data file into a graph for your analysis and you can post it here for others to comment on.

The things you've done and the parts you've changed so far are good things for any older vehicle, but it's time to do the tests to give you the actual answer so you can address the actual issue.

Good Luck

 

That sounds like a good decision. You've done what you are comfortable doing and a new cat sounds like a good solution. Being that the one on the car now is an aftermarket, and they are notorious for going bad fairly quickly, a new cat will likely solve your problem at least in the short term. The more expensive aftermarket cats will typically last much longer than the cheap cats so keep that in mind when you're looking. BTW, have you put enough miles on the car now after the oil change to see if you have an oil consumption problem? In your reddit post you said you only put on 150mi since you did your work and I suggested you monitor your oil consumption to see if you're burning oil. If you're burning a lot of oil you can expect a short life for a new cat. But a reasonably good aftermarket cat (not the real cheap ones) should at least get you past the inspection test in Oregon. Good luck

 

There are two O2 sensors. Change the other one.

 

Can you please share the spacer info? I can’t find them on eBay and need to do this as well. Thank you.

 

or you could go my route, I was starting to throw the same codes added cat cleaner and that kicked the can down the road. Then I parked it by the train station in Chicago and it was stolen. Now I have a new cat and no more codes....