Catalytic Converter Temps

I'm thinking those PIDs would be the temperatures of the heaters inside the sensors. I don't think there is any sensor for the catalyst temperature. (And even the heater temps must only be estimated from the voltage and current.)

 

Those temps seem fine, the second temp should always be lower if not then your cat is going bad.

Ask me how I know...

Anyway below is a picture of my new Magnaflow cat temps.

 

I've owned other cars with actual exhaust gas temp sensors- real thermocouples feeding real temperature data into the computer. This is what leads me to believe that ChapmanF is correct- the temp indications you see aren't quite real, they are derived from other data at the oxygen sensors.

It may not be fair game to use those temp readings to diagnose anything other than the sensors themselves. Typically an oxygen sensor is heated by 12v power from startup until they receive enough heat from the exhaust stream to work without external power, so it is useful to be able to validate whether a given sensor is getting hot enough.

I'm not sure about Toyota's setup, but generally the logic for a P0420 monitor is such that it compares the free oxygen coming out of the catalyst to the oxygen level going in. Roughly, if there is still oxygen on the output, then chances are the catalyst wasn't doing its job. Of course that is subject to sensor error, but there you have it.

 

The oxygen sensors themselves age, with the primary problem being longer response times.

I have replaced the upstream sensor between 125k and 150k miles on several other cars I've owned in the past decade, and in all cases there were slight improvements (recoveries) in MPG and acceleration. None had made it to the point of showing P0420 though.

My logic was that if the timing (input data latency) stays tight during closed-loop operation, I'll benefit directly from fewer air/fuel ratio errors. A secondary benefit is that this leaves less cleanup work for the catalyst to do, so it can last longer.

Our Prius only has 70k, so it hasn't come up yet and I haven't done this on any other Prius either.

I've never needed to replace the downstream sensor on any car I've owned.

 

B1S1 is the air-fuel ratio sensor right in the exhaust manifold before the cats. B1S2 is the oxygen sensor downstream of the first catalyst and before the second catalyst.

Toyota's monitor is explained in the troubleshooting section for P0420. It's kind of interesting. It is an active test for Oxygen Storage Capacity in the catalyst.

At some moment, as you motor down the road, the ECM will decide to do this test. It will adjust the fuel trim a bit down to the lean side for a short period of time, leaving extra oxygen in the exhaust, which the catalyst will store up.

In the second half of the test, the ECM drives the fuel trim to the rich side, and watches the downstream oxygen sensor (B1S2). Eventually, once the oxygen stored up in the catalyst in the first half is all used up, that reading will go rich. The longer that takes, the higher the oxygen storage capacity, and the better the catalyst's condition. Then the test is over and the ECM goes back to using the normal calculated fuel trim.

When the measured OSC comes out low enough, P0420 is set.

 

my pre-Prius experience is that slightly reduced MPG, reduced acceleration, and larger fuel trims (larger corrective swings) as seen on a scangauge were the biggest symptoms.

Mind you I'm not relating anything to do with appropriate procedure for dealing with P0420 on a Prius, this is strictly background info.

For your situation I'd be working straight off the tech info repair manual.

 

My P0420 checklist:

NO exhaust leaks from the cylinder head to 18" past the downstream O2 sensor- not even a pinhole.

Engine running good. NO misfires. Gets to full operating temperature. Burning lots of oil will kill the cat.

A/F sensor (upstream) ok- reads around 3.3V at steady throttle. Goes down toward 2.0V during snap throttle heavy accel (rich), and to 5.0V during decel fuel cut(lean). Ideal total fuel trims at 0 +/- 5%. At steady throttle, short term fuel trim is fairly steady- moving about 1%.

O2 sensor (downstream) ok- reads 0.9V (rich) during multiple snap throttle-accel, and near 0V (lean) during fuel cut. Usually around (steady-ish) 0.7V during steady throttle with a working cat.

If those 4 things check "good", the car needs a new cat.

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