Re: P0420 code

Does this mean I may only need to replace the O2 sensors and not the catalytic converter. Wouldn't this be a less expensive option to try before replacing the converter?

The proper diagnostic procedure is to CHECK both O2 sensors to see if they are functioning properly before replacing the catalytic converter. If they appear to be functioning properly, then the catalytic converter is suspect, and the dealer is probably replacing both O2 sensors to make sure the car doesn't come back with the same trouble code.

The dealer never said anything about the sensors. They just checked the code and said it was a catalytic converter and quoted 2600.00. I just sent the owner of the shop a e-mail requesting a quote for the O2 sensors. I think I may get a second opinion. There is new shop across the street from my normal shop.

How many miles on the car?

The car has 115K.

Sorry, you're out of luck on the emissions warranty. If you want to invest in a volt/ohm meter, you can check out the O2 sensors yourself and see if they are working properly or not.

What should the readings be?

Here is a pretty good explanation of how O2 sensors work and what the voltages should be:

Great article-- thanks Ray. I have a 1992 SC400 with 190K miles. The O2 sensor has never been replaced but I have no symptoms mentioned in article. Should I consider to replace as preventative measure?

Appreciate your prolific feedback to this forum.

If it were my car, I wouldn't worry about it, but if you want, you can measure O2 sensor voltage and replace if it is only barely within specs.

Thanks -- good reference!

You're welcome!

P0420 is usually reliable (sorry.)

O2 Sensors have to respond reliably and consistently to their tests to set this.

cyl 1, 2, 3 => B1 cyl 4, 5, 6 => B2

Look for aftermarket cat. ... they exist.

The dealer is supposed to check for:

1. Make sure the engine is well tuned. If the ignition system is putting out too much hydrocarbon it will degrade the converters.

1. Check for false air entries in the exhaust system. This can throw off the oxygen sensors.

2. Oxygen sensors themselves. This is almost impossible as the sensors can be marginal or even fail and they won't set codes. Being wear items, I would replace the sensors. In fact, I've fixed a P0420 problem by junking the Denso sensors and using the Bosch Planar type oxygen sensor. Bosch planars are best-in-class.

1. Then you should consider changing out the converters. Converters are substrates coated with platinum. Unless they crumble you should be fine.

$2600. Sheeeeeesh.

Air entering the exhaust system will probably set DTC P0125 - insufficient coolant temperature for closed loop operation.

Checking the oxygen sensors themselves is very easy with a volt meter or OBD II scan tool with the capability to display raw sensor output values.

Being wear

I recommend checking components before replacing them, even if they are wear items. For example, people would not be happy if a shop replaced brakes or tires without first checking their condition to see if they need replacement.

In fact, I've fixed a P0420

If the catalyst becomes coated with particulate matter, the exhaust gas does not come in contact with the catalyst and so the catalyst doesn't do anything.

False air entry into the exhaust is usually seen as a lean condition (excess presence of oxygen) and will cause ECU to command more fuel. Why would it set P0125?

I've seen sensor waveforms. I'd think the reaction time waveform need to correlate with fuel to diagnose slow sensor (and not dead sensors), which is the problem with many P0420 codes. I've seen people done it using oscilloscopes with snap-throttle tests, but not sure about the OBD-II data frames, so I'll leave it to the experts here.

"...while a dead sensor can be detected with a relatively inexpensive digital volt-ohmmeter, a slow sensor can only be diagnosed by a more expensive oscilloscope or scope meter." - Bosch USA

Agreed. That why cars need to be well tuned, which is the #1 thing I mentioned techs should check. But unfortunately, this Toyota (Lexus) tech didn't even do that, recommending converter changed just based on a P0420. That's too bad.

Yup, that's why it's so important to keep the engines well tuned!!

Yes, air entering the exhaust will make the exhaust seem lean, and I don't know why the ECU sets P0125. For some reason, the factory repair manual does say to check for an exhaust leak if P0125 comes up, and I have seen this code caused by exhaust leaks, usually from a loose O2 sensor.

A higher end OBD II scanner can compare O2 sensor output. To set P0420, the ECU compares the signals from the #1 and #2 sensors. When they are working properly, #1 sensor should have both higher frequency and higher amplitude than sensor #2 because if the cat is working, sensor #2 shouldn't see as much activity. If the signal from sensor #2 looks like the signal from sensor #1, the ECU assumes that the cat isn't working and sets P0420. If sensor #1 is marginal and not reacting quickly enough, its signal starts to look like the signal from sensor #2, and if the signals look alike, it will set the code.

I don't know whether or not the tech checked the signals from the sensor, and if the sensors were working properly, then the cat is bad. If the tech checked the sensors, then he should have noted that on the repair order, although this is not always done. If the tech did not check the sensors, then that is incomplete or lazy diagnosis.

I agree, engines should be kept well tuned. Fortunately, modern engines require very little tuning. Unfortunately, owners often neglect the engines since there is so little to do.

Yeah. People have said the diagnostics codes are often wrong and one has to make sure the root of the problem by additional tests. Toyota's setting P0125 for false air entry and how P0420 sometimes mis-dignoses the catalytic converter problem are good examples.