CO failure

[brianmen]

Hi,
I have aan Audi A4 1.6L 2003 Model.

Few days back, why driving my car i found the Engine light on the dash board pop up.
i immediatly drove the car to the nearest garage to check the fault code.
The guys at the garage found code P-0420 for the cause of the engine light to pop. they said they found no other error code than that.
they carried emmision test and found CO fail. they did twice FAST IDLE TEST and found the value to .396 & .398. the printed copy says the limite value to be .300. so i assume it is a little way high.
The mechanic said i might need to have catalytic convertor and lamda sensor replaced to fix the issue.
My question is

1. would this be real cause for the engine light to pop or there could be any other issue?
2. do i need to change catalytic convertor and lamda sensor to fix the issue or there is any other alternative. the reason i am asking is the mecahnic said the entire replacement would cost me roughly £600 which is expensive.
3. can you please guide me if there is any cheaper alternative

The mechanic said he has disabled the sensor for the code, the engine light stopped popping for a day after that. but it has srated reappering once again

your comments are appreciated

[bez101]

details for your code
[hide]

• 1 16804/P0420/001056 - Catalyst System; Bank 1: Efficiency Below Threshold
  
  • 1.1 Possible Causes
  • 1.2 Possible Solutions
  • 1.3 Special Notes

16804/P0420/001056 - Catalyst System; Bank 1: Efficiency Below Threshold

Possible Causes

• Leakage in Intake and/or Exhaust System
• Catalyst faulty
• Oxygen Sensor(s) faulty
• Oxygen Sensor(s) Control faulty

Possible Solutions

• Check Intake and Exhaust System for Leaks
• Check Catalyst
• Check Oxygen Sensor(s)
• Check Oxygen Sensor(s) Control
  
  • Perform Oxygen Sensor(s) Aging Check

Special Notes

• For 1.8T's experiencing this fault code, this thread may be useful: http://forums.vwvortex.com/zerothread?id=663057

i had this in a 1,6 A3 was a 02 sensor