2018 Cadillac P0420: What It Means & Is It Safe to Drive?
P0420 on the 2018 Cadillac: 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.
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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 2018 Cadillac
- 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 2018 Cadillac, repairs for P0420 typically fall between $400 and $2400, depending on the root cause and labor rates in your area.
Known issues on Cadillac
Cadillac's 3.6L LGX and LFX engines share the P0420 tendency with other GM 3.6L vehicles. Verify oil consumption levels and check for exhaust manifold warping on high-mileage models.
Most commonly affected models
CTS 2014–2019XT5 2016–2020SRX 2010–2016