<!-- from fixya rte -->Turn off emissions warning light - 2004 Honda Accord

Blocked or defective cat or the exhaust sender unit is faulty

You must have the system computer scanned for the fault code or codes that are stored when the powertrain control module turns the light on. The with the fault known you perfrom the listyed diagnostics to cure the issue.

As we tell everyone else who comes on here with warning lights: When there's a warning light on, that means a trouble code is stored in the computer. Don't ask us to guess what caused the light to go on when all you have to do is get the trouble codes read.

you need to pull the codes and go from there

sounds like a faulty air mass metre or lambda sensor throtle position sensor depending on which year or model id get its fault code read

It may be the computer itself. If the light keeps returning, either the computer is faulty or the fans are really failing. I do not recommend dealers but actual mechanic shops.

I bought a 2000 polo 1.4 yesterday after having an MOT. The warning lamp came on today but the car is running fine. Whether its the Lambda sensor or just the light itself is a common fault, I think garages turn off the lights to get it through the MOT then they come on again in a day or two!

it will not pass the test with the light on, and if u turn off the light they will know when they test it by testing/checking the system readiness function. Go to a Kragen or an Autozone they will run the test to see what trouble code has turned the light on, it is free, post the code back here. Oh have then reset the light after the test.

don't buy German engineering it's very complicated to fix