2008 Audi P0420: What It Means & Is It Safe to Drive?
P0420 on the 2008 Audi: Catalyst System Efficiency Below Threshold (Bank 1). Here's what it means, what usually causes it, what repairs typically cost, and whether it's safe to keep driving.
What this code means
The downstream oxygen sensor detects that the catalytic converter is not storing or converting exhaust properly. This is one of the most common check engine codes.
Common causes on the 2008 Audi
- Failed or aging catalytic converter
- Faulty downstream oxygen sensor
- Exhaust leak before or after the catalyst
- Engine misfire or rich/lean condition damaging the catalyst
Symptoms
- Check Engine Light on
- Often no drivability issues
- Possible failed emissions test
- Rotten egg smell in some cases
Typical fixes
- Replace catalytic converter
- Replace oxygen sensor(s)
- Repair exhaust leaks
- Fix underlying fuel/ignition issues first
Repair cost range
For a 2008 Audi, repairs for P0420 typically fall between $400 and $2400, depending on the root cause and labor rates in your area.
Known issues on Audi
Audi P0420 on EA888 2.0T TFSI shares the VW architecture—the catalyst is part of the turbo downpipe assembly and requires removing the downpipe to replace. On the B8 A4/A5 with cam follower wear (a known issue on high-pressure fuel pump cam followers), erratic fuel delivery can cause rich/lean swings that damage the catalyst. On quattro AWD models, access to the downpipe requires unbolting the subframe on some configurations—budget $800–$1,500 in labor alone. Confirm the wideband O2 sensor (upstream) calibration is correct before condemning the cat.
Most commonly affected models