Code p0420 acura m'dx - 2005 Acura MDX

I do not know what these codes are but as a starting point look them up on a web search

When it returns. P0420 is a bad catalytic convertor. Nothing else will set this code. It will need to be replaced.

P0420-catatytic convertor failure Replace convertor
P0401- EGR flow insufficient-Clean EGR passages
P0730-Faulty transmission shift control-Get diagnosed

DTC P0420: Catalytic System Efficiency Below Threshold
You will have to replace the catalytic converter nearest the firewall.

in the exhaust pipe near the front of the cat converter.

OK, If I understand your question correctly, Your Check Engine light is coming on and you are getting a Diagnostic Trouble Code (DTC) P0240 and we are talking about a 2000 Honda Civic DX with a 1.6L engine? If this is not correct, please let me know.

DTC P0240 is not a valid code for the vehicle listed above. Are you sure we are not talking about DTC P0420? If this is the case, then your catalytic converter is not working and most likely will need to be replaced. If you previously had fuel system problems, this is what most likely destroyed your catalytic converter - especially if the vehicle was driven fo some time with the fuel system problem. It is VERY common for this code to come up after other engine performance problems have been repaired. The reason for this is because the computer will not run the catalyst monitor if there are other problems in the ignition or fuel systems. So as long as these other problems exist, the computer has no idea what the condition of the catalyst is. Once the other problems are fixed, then the catalyst moitor will run. If the computer finds that the catalyst has been damaged and is not performing as designed, it will set DTC P0420.

The P0420 code is set when the oxygen sensor after the catalytic converter gives a reading the same or very close to the reading of the oxygen sensor(s) before the catalytic converter.

There are usually three possible causes...
The front oxygen sensor (before the cat) is giving an incorrect reading.
The rear oxygen sensor (after the cat) is giving an incorrect reading.
The catalytic converter is not working properly.

Since you recently replaced the cat, the logic points to the sensors, but it could be the cat again. Aftermarket converters are usually smaller than OEM and don't last as long.

For a more thorough description of the code and troubleshooting visit
http://www.obd-codes.com/p0420

Our Subaru encountered this issue several years ago under warranty. The first time the dealer replaced the front oxygen sensor. This was the cheapest repair and since it was an intermittent problem I am sure they hoped to delay the issue until the warranty was up. The second time they replaced the catalytic converter and we haven't had any more issues.

err 3 is not an engine system fault code, a tyupical code looks like this P0420, this happens to be a evaporative emission system fault code.

P0420: Calayst system efficiency below threshold (Bank no. 1)
You can clear the code by turning your engine off, locating fuse number 12 in your under hood fuse box (backup, radio 7.5A) and pulling that fuse for about 10 seconds. If the code comes back, it was not a glitch and I would have the code checked out (has to do with you catalytic converter, I believe).
Hope this helps you.

ur p0172 code refers to ur oxygen sensor, most likly the rear .. and the p0420 code is a catalytic converter deterioration code... the p0172 and 420 can be related because that oxygen sensor is the one installed in ur cat converter.....u most likely will need to replace both the cat converter and oxygen sensor