Have you considered the O2 sensors? The downstream O2's measure the efficiency of the catalytic converter. That's the ONLY way it can tell if there is a problem with it. There is nothing in the cat itself.

You may not get a code either.

If there was a problem ahead of the cat you should have "fuel trim" codes.

I'd start with them and if that doesn't work you're probably looking at new cats.

 

Okay, I'll bite. How do you check sensors?

 

It is nearly impossible to test an O2 sensor with a volt meter, they switch so fast the meter can not give you an accurate reading. The best way to check an O2 yourself is with a scan tool that reads live data and fuel trim. There is a lot to it but you have short term fuel trim and long term fuel trim data for each bank of the engine. In an ideal situation the short and long term fuel trim should equal out to zero, +1 or -1 for each side. When I had to replace my own cats my fuel trim in the right bank was way off, it was like -7 and +3. I could tell the cat was bad because I was running super lean on that bank. The lean condition may have been due to using Marvel Mystery Oil in my gas. I used it for about 18 months trying to get better gas mileage. I stopped using MMO after my cat went bad. I had to do a lot of online research to gain a very rudimentary understanding of fuel trim and what the PCM is looking for in the air/fuel ratio. I bought a good handheld scan tool at Harbor Freight for about $65.00 with a coupon. It reads fuel trim, O2 and live data and it really helped me diagnose the cat problem. My scanner has come in very handy on other problems too. A good exhaust shop or independent garage can check your exhaust and O2 data too before recommending any work to be done.

Jimmy

 

Good explanation Jimmy, thanks.

 

I like that explanation. Harbor freight here i come...

 

I did find this...

A code P0420 may mean that one or more of the following has happened: Leaded fuel was used where unleaded was called for A damaged or failed oxygen sensor (HO2S) Downstream oxygen sensor (HO2S) wiring damaged or connected improperly The engine coolant temperature sensor is not working properly Damaged or leaking exhaust
manifold / catalytic converter / muffler / exhaust pipe Retarded spark timing The oxygen sensors in front and behind the converter are reporting too similar of readings Leaking fuel injector or high fuel pressure Cylinder misfire Oil contamination

Read more at: http://www.obd-codes.com/p0420
Copyright OBD-Codes.com

Some suggested steps for troubleshooting a P0420 error code include: Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required. Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady). Inspect the downstream heated oxygen sensor (HO2S), replace if necessary Replace the catalytic converter Overall probably the biggest mistake vehicles owners make when they have a P0420 code is to simply replace an oxygen sensor (H02S). It is important to do proper diagnosis so you're not wasting money replacing parts unnecessarily. We strongly recommend that if you need to replace the catalytic converter that you replace it with an OEM unit. Second choice would be a high-quality replacement part. There are many stories in our forums where folks replaced the cat with a cheaper aftermarket one only to have the code return shortly thereafter. One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

Read more at: http://www.obd-codes.com/p0420
Copyright OBD-Codes.com