Hi All,

This morning my Check Engine Light came on (with nothing noticeable while driving or idle speed, etc). I finally got over to Checker this evening and pulled the code and got a P0420: Catalyst system efficiency below threshold (Bank 1). From the last snapshot, the STFT and LTFT were both 3.1% at about 1800 rpm. So, after searching this site and others, it appears this code could be from a few causes:

Bad rear 02 sensor
Bad cat or leaks in pipe around cat
Coolant temps

The front 02 sensor was already replaced a couple of years ago. A few months back I got a Magnaflow exhaust and a hi-flow (aftermarket) cat that was welded up by a local shop with a good reputation. Not fully understanding how the OBDC diagnostics work, it seems fishy that the STFT and LTFT were the same, but they also appear reasonable (I found write ups that said to ignore LTFTs below 10%). So, tomorrow morning I'll probably check my coolant and hunt around the exhaust pipe. If anyone has any advice on how best to proceed, it would be greatly appreciated!

I don't think a cat that is a couple months old is going to die.

If you used an o2 sensor from the dealer, there is almost no chance that it is bad by now, meaning the mixture should be controlled accurately so as to not poison the cat.

The advice you picked up about coolant temp should have you looking for the engine not warming up.

So it is likely it could be the rear o2 sensor.

3% fuel trim ... your intake is well sealed, maf reasonably accurate and injectors working well. If you had a leak in this part of the exhaust you would hear it.

With an OBD2 tool (I really like PCMSCAN) you can read the actual o2 sensor voltages in real time. Do not hook up a meter probe to the o2 sensor connections!

eleet,

Thanks for the advice and rec on the scan tool. I think you are right about the rear 02 sensor: the truck drives and idles just fine, its just that the light comes on, right at start up. As far as the 02 sensor voltages, are the correct values easy to find on-line (I have not looked yet)?

OK, another important point is that my fuel economy has dropped over the past months. There is another current thread on this topic, in which I replied that making sure my tires' air pressure was around 45 seemed to make a big difference. But, even with that checked (and the plugs/wires being relatively new, the air filter and throttle body being cleaned, etc), I am still at least 1 mpg under average. So, this recent P0420 code sounds more and more like a bad 02 sensor that has had me running suboptimal and now finally bad enough to throw the code. I know that I need to try and check the voltage cycling to see if the sensors are within spec, but in general, would a drop in fuel economy point to only the front sensor being bad, or could the rear be involved too? From what I understand, the rear sensor is there just to make sure that the cat is restricting flow, and it does not send anything back to the ECM with respect to air/fuel ratios. The front sensor was replaced 4 years ago and only has 20K on it. The rear sensor is stock.

Thanks for the education!

Is this a factory O2 sensor or one of those Bosch universal ones?

Unfortunately I do not know. It looks exactly the same, and it was done at the Toyota dealer (I pay a mechanic there under the table for 1/2 price labor and parts at cost!), so I can only assume its OEM.

Probably OEM sensor, given where you got it installed. Factory o2 sensor will have a toyota or denso part number on it. It will also not have any splices in the wire.

The front and rear o2 sensors do different things. The one in front before the cat regulates the mixture to whatever is programmed in the ECU for current rpm/load/throttle. The one in the rear, behind the cat, simply indicates whether the cat is working so it can light the check engine light if it's not.

I don't know what the o2 voltage readings should be. I have not observed them while my truck was running: PCMSCAN periodically reads all the values you specify, and if you ask it to keep track of more things then the period becomes longer, reducing the number of data points in your log for a given time. So, I only check the ones I'm interested in.

Out of the Taco fsm they should be 11-16 ohms for the 3.4 sensors.

just a question on the topic of o2 sensors, how many are there on the t100? I know there is one behind the cat and one right before, are there anymore on the truck?

just the two...