My buddies 03 Accord failed emissions, inspector said it was a sensor or cat.
I just scanned the car with my OBD Link scanner and pulled a P0420, which indeed seems it could be a sensor or the cat.
The car has about 160k on it.
Are there other possible culprits, can I test the O2 sensor, what should I do?
The guy gave me his 98 Accord, so I'm trying to help him out, plus he doesn't have much money to put in the car.
Thanks!

 

Does your scanner read live data?

 

It does to an extent, which upon doing a quick search at his house on my phone seems I could pinpoint it but it was pouring rain and I'd have to play with it a bit.
I think it was reading O2 sensor output, not sure if mine does that, it can read RPM's, speed, and other things real time, but I've used it little for more than code pulling/resetting so I'm not sure.
I was thinking I'd swing by there this weekend but just remembered I'll be out of town, so probably another evening this week.
What should I look for, and can I verify with my multimeter if it is the O2 sensor?
Thanks!

 

The O2 sensor output would be good to look at on both sensors. Also fuel trims.

There is a good diagnostic youtube channel called schrodingers box where he covers a ton of issues like yours. I've watched all his videos and they can help you get an idea of what to do. There are also a lot of people on here that can provide advise once you start getting more information.

 

I watched a few of the videos of his, and others about P0420 code.
I had reset the codes last time I scanned his car and told him to see how long before the CEL came back on. It took about 30 miles before CEL came back on, stayed lit for about 100 miles and then went off.
When I rescanned a few minutes ago there was no CEL, but a P0420 code stored.
So I looked to see what the C02 sensors were doing, sensor 1 wasn't reading anything, sensor 2 was fluctuating quite a bit, which from the videos sensor 1 should fluctuate a bit and sensor 2 should be pretty steady.
So, hopefully sensor 1 is before cat, 2 after, and I'm guessing 1 is bad.
Anything further to check, or way to verify the sensor is bad?

 

What sensor output were you reading on the scanner? Fuel trim, voltage, etc.. Also, what values were you getting?

 

The app was indicating 02 sensor 1 & 2. Sensor 1 was reading 0.00, and sensor 2 was fluctuating between 0.13 - 0.80 or so.
My understanding is the sensor before the cat should be doing the fluctuating and the one after the cat low and steady as the computer adjusts the air/fuel ratio.
Video examples showed results based upon a bad CC, I wished they had said/shown results with a bad sensor.

 

Sensor 1 on new cars doesn't give a fluctuating voltage like the older O2 sensors. Sensor 2 is an older style O2 sensor.

The scan tool should have fuel trim data that will let you know if the computer has to add or subtract fuel. You will have to look at the long and short term fuel trims based on sensor 1 to see what information is being proved to the computer. On a perfectly running engine, the fuel trim should be close to zero. An older engine should be within 10%.

 

So what does an 2003 count as?
Sensor 1 wasn't showing any voltage, I'm intrigued and confused by your reply.
If sensor 2 is an older style, I would have to presume because the rest of the system is as well, unless you're implying an older 02 sensor was installed on the car?
Regardless of fuel trim, shouldn't sensor 1 have some voltage value if its good?

 

Your 2003 (new car from my description) will have a new wide-band Sensor 1, and a normal oxygen Sensor 2.

Sensor 1 is a wide-band sensor (aka lambda sensor). The wide band sensor sends a different signal to the PCM than the older kind of oxygen sensors. The signal is a slight change in amps (or voltage, can't remember). Regardless, you won't be able to read it with a volt meter.

Remember, you are reading data interpreted by the engine computer on your scanner. The voltage is not important for this sensor.

Sensor 2 in the catalytic converter is an old style oxygen sensor and should not be a wide band sensor. Sensor 2 only monitors the catalytic converter performance and does not factor into the air/fuel mixture for gas mileage and emissions.

That is why I suggested reading fuel trims.