P0420: Catalyst System Efficiency Below Threshold (Bank 1)
TL;DR
P0420 = catalytic converter efficiency too low on Bank 1. Severity: medium. You can usually keep driving short-term, but fix it before an emissions test. Top causes: worn catalytic converter (55%), bad rear O2 sensor (30%), exhaust leak (10%). Typical repair cost: $150 (O2 sensor DIY) to $1,400 (converter at a shop).
Can I keep driving with P0420?
IF the check engine light is steady (not flashing) → you can usually drive normally for days to weeks; the risk is failing emissions and slightly worse fuel economy. IF the light is flashing OR you also have misfire codes (P0300–P0308) → stop driving and fix the misfire first, because raw fuel can destroy the converter and turn a cheap fix into an expensive one.
Symptoms
- Check engine light on (steady)
- Slightly reduced fuel economy
- Failed emissions / smog test
- Occasional sulfur ("rotten egg") smell from exhaust
- Usually no noticeable change in how the car drives
Top causes (ranked by probability)
| Likely cause | Probability | Notes |
|---|---|---|
| Worn or failing catalytic converter | 55% | Most common on vehicles over ~100k miles |
| Faulty downstream (rear) oxygen sensor | 30% | Cheapest thing to check first |
| Exhaust leak near the sensors | 10% | Lets outside air skew the reading |
| Unresolved engine misfire or rich/lean condition damaging the converter | 5% |
What does P0420 mean?
Technical explanation
The engine control module compares the oxygen-content signals from the upstream (pre-catalyst) and downstream (post-catalyst) oxygen sensors. A healthy catalytic converter stores and releases oxygen, so the downstream sensor signal should be much flatter and slower than the rapidly switching upstream signal. When the downstream signal begins to mirror the upstream signal, the ECM concludes the converter’s oxygen-storage capacity has dropped below the calibrated threshold and sets P0420 (Bank 1 = the cylinder bank containing cylinder #1).
In simple terms
Your exhaust has a part called a catalytic converter that scrubs pollution out of the fumes. Two sensors check the air before and after it. When the "after" sensor sees almost the same dirty air as the "before" sensor, the computer knows the scrubber isn’t doing its job and turns on the check engine light with code P0420.
How to diagnose P0420 (step by step)
- Scan and record all codes. Use an OBD-II scanner. If misfire (P030x) or fuel-trim (P0171/P0174) codes are present, fix those first — they cause P0420.
- Check for exhaust leaks. Inspect the manifold, gaskets and pipes near both O2 sensors. A leak upstream of the rear sensor can fake a P0420.
- View live O2 sensor data. Compare upstream vs downstream sensor voltages. A downstream sensor that switches as fast as the upstream one points to a dead converter.
- Test the downstream O2 sensor. If the sensor is sluggish or unresponsive, replace it before condemning the converter.
- Confirm converter health. Measure backpressure or use a temperature differential check across the converter before replacing it.
Repair options & cost
- Replace the downstream oxygen sensor (try first — cheapest)
- Repair any exhaust leak near the sensors
- Replace the catalytic converter (OEM or CARB-compliant aftermarket)
- Fix the root cause (misfire / fuel trim) before replacing the converter
| DIY cost | $40–$250 |
|---|---|
| Workshop cost | $200–$1,400 |
| Repair time | 30 min (O2 sensor) to 2 hours (catalytic converter) |
Costs are local ballpark ranges and vary by region and vehicle.
Tools you’ll need
- OBD-II scanner (BlueDriver / ANCEL)
- O2 sensor socket set
- Penetrating oil
Vehicle-specific notes
- Toyota / Lexus: factory converters are durable; P0420 here often turns out to be a real converter at high mileage.
- Subaru: notorious for genuine converter failures triggering P0420 — check for TSBs and extended warranties.
- Honda: frequently caused by the rear O2 sensor; replace the sensor before the converter.
Common mistakes to avoid
- Replacing the catalytic converter before ruling out a cheap rear O2 sensor
- Ignoring an underlying misfire — it will destroy the new converter too
- Fitting a cheap non-compliant aftermarket converter that re-triggers P0420 within weeks
- Clearing the code without fixing anything (it returns after the next drive cycle)
Frequently asked questions
Can I drive with a P0420 code?
Yes, usually for short-term driving, as long as the check engine light is steady and there are no misfire codes. You will likely fail an emissions test until it is repaired.
Will a new oxygen sensor fix P0420?
Sometimes. If the downstream O2 sensor is the cause (about 30% of cases), a new sensor fixes it for under $250. If the converter itself is worn, the sensor will not clear the code.
How much does it cost to fix P0420?
From around $150 for a DIY oxygen-sensor replacement to roughly $1,400 to replace a catalytic converter at a workshop, depending on vehicle.
Is P0420 serious?
It is a medium-severity emissions fault. Your car runs but pollutes more and fails smog tests. It becomes urgent only if combined with a misfire, which can damage the converter.
P0420 summary
| Meaning | Catalytic converter efficiency low (Bank 1) |
|---|---|
| Severity | Medium |
| Safe to drive? | Yes, short-term (unless misfiring) |
| Top cause | Worn catalytic converter (55%) |
| DIY cost | $40–$250 |
| Shop cost | $200–$1,400 |