'96 Camry P420 catalyst code?

[Miescha]

77k miles on the 1996 camry with 4-cyl engine. Runs great. Complete baseline of fluids, plugs, wires, cap, rotor and new front axles in past 2k miles since purchase.

CEL came on and it reads P420 so it seems either the cat needs replacing or one (both) O2 sensors. I have access to a code reader that will display data on the sensors, anyone know what I need to look for to determine if it is the sensors or the cat?

Anything else I should be checking?

Thanks!

 

[Rix86]

If the rear sensor is giving a waveform similair to the front sensor, the cat is done, no longer "working".

 

[Miescha]

If the rear sensor is giving a waveform similair to the front sensor, the cat is done, no longer "working".

I won't get to use the code reader until tomorrow morning, and I don't know all the data that it provides for the sensors, so this may become obvious at that time - but in the meantime, what is a 'waveform' of the sensor? What exactly should I be looking for?

EDIT: Never mind, I think I found it. Here is the description I found:

When the HO2 sensor is at operating temperature, the voltage output of the sensors is 0.45V at the correct Fuel/Air ratio. Higher voltage than this indicates a rich mixture. The higher, the richer. Lower voltage is leaner. The output voltage of the HO2 sensors should oscillate around 0.5 V while operating normally.

The voltages you found (if they were mean voltages) mean that the oxygen content has gone down by passing through the Cat Converter. Since the catalyst is an oxidation catalyst (using up oxygen), this makes sense. If they were peak voltages, then it is meaningless. Peak voltages are a function of the frequency and the O2 content.

The comparison made by the ECM is not relative voltages (since they constantly change owing to the design of the HO2 sensor), but waveform frequencies. The frequency and peak voltage of the upstream [bank 1 sensor 1] sensor should be higher than the frequency and peak voltage of the downstream [bank 1 sensor 2] sensor. If the frequency is pretty much the same, then the catalyst performance has deteriorated.

The HO2 sensors are tested by measuring their resistance. The resistance between terminals +B and HT should be 11-16 ohms cold, or 23-32 ohms at operating temperature.

You should be able to measure the frequency of the waveform from both HO2 sensors (thus checking catalyst function) by connecting a voltmeter (0-1 V scale) or oscilloscope to terminals OX1 [bank 1 sensor 1], OXS [bank 1 sensor 2], and E1 of the ECM connector. Check the terminal voltages/waveforms while running at 2500-3000 rpm. The engine should be first warmed up at idle until hot, and then heatedby running it at 2500-3000 rpms for 3 min.

Note: This discussion does not apply to an engine with an Air/Fuel ratio sensor instead of an HO2 sensor.

 

[Dynosoar]

Change the spark plugs this is common on a Toyotas with a missfire.

 

[Miescha]

Change the spark plugs this is common on a Toyotas with a missfire.

Thanks for the suggestion - it does seem a common problem and I only wish it were mine.

Easily missed in my original post, but I have new spark plugs in already (no code before and no code until ~2k after plugs changed). Full details of car history here scroll down to post #8 for what I've done (add front axles, timing belt and water pump to the list).

I bought the car used from a friend (he was second owner). Only 75k miles as his daughter rarely drove it. In fact, I've read a few threads that suggested continual short-distance trips may actually harm the cat over time. The theory was that if you only drive 5-10 miles at a time and the engine cools completely between trips then the engine never gets to full operating temps and the cat also never gets to full operating temps. I really don't see how this would cause the cat to fail early (and I'm not sure 12 years and 75k miles is early either).

I did test the front and rear O2 sensors and they seem to be working correctly (front is very active, rear is almost inactive). My dealer also confirmed that the sensors were working correctly based on the data print-out I showed him (but of course warned that the data can change quickly under different conditions). I did drive the car to get to standard operating temp and then tested with the car parked and engine rpm at 3000. Engine and coolant temp sensors also tested correctly. The dealer did mention that a rich fuel mix could also cause the PO420 code, but didn't have time to elaborate on why or how or what needed to be done to fix this. Obviously, at the time of my testing the fuel mix was normal. In the end, the dealer said they normally see the PO420 when the cat is gone and warned that it was a pricey part.

I'm waiting to hear, but I'll certainly be calling CDan (I need to order other stuff anyway). If it is too high, I guess I could go aftermarket, but I do wonder how long a cheap cat would last.

FWIW, I did tap on the cat with a rubber mallet a few times and didn't hear anything rattling around. Other than that, I really don't know what a 12 y/o camry cat is supposed to sound like.

My plan at the moment is to just let my daughter take it back to school in Madison, WI and if the CEL reappears then I'll address it again when she comes home next (Thanksgiving?).

Thanks all for the advice.

 

[concretejungle]

may be a little late on this response, but i get that same code on my cruiser. I had a leak in the exhaust at the flanges between the two sensors. I did not have bad cats even though Toyota really wanted to replace them. A $5 gasket and i was fixed.

 

[Miescha]

Thanks Concrete - its never too late for good advice!

Actually, pretty timely too. The code disappeared for over a year and then came back around Thanksgiving this year so I looked at it again while she was home the past week for the holidays. There is zero smell from the exhaust (not that I would smell anything anyway) but the fancy smancy diagnostic reader still shows both sensors working correctly.

I didn't look closely at the gaskets, and she is leaving tomorrow morning (and its really freakin' cold here by STL standards which means around 8 degrees F) so maybe I'll run it by an exhaust shop early in the morning as see if they can just take a 'quick look' as I don't want to wrench on it in the cold.

Then again, maybe I'll just reset the code and see how long before it comes back on - maybe I'll make it to next year