Toyota Check Engine Code
P0420 on Toyota
Catalyst System Efficiency Below Threshold (Bank 1)
Also covers: Lexus, Scion
P0420 on Toyota: what makes it different
P0420 is one of the most common codes Toyota owners face — and on Toyotas, it usually means the catalytic converter is degraded, but the front O2 sensor is the cheaper thing to replace first. Toyota cats are unusually long-lived (typically 200k+ miles) but California-emissions models have legal complications.
Most-affected engines
- 2AZ-FE 2.4L (Camry, RAV4, Solara)
- 1MZ-FE 3.0L V6 (Camry, Avalon, Highlander, Sienna)
- 2GR-FE 3.5L V6 (Camry, Avalon, Highlander)
- 1NZ-FE 1.5L (Corolla, Echo, Yaris)
- 5VZ-FE 3.4L V6 (Tacoma, 4Runner)
Common model years: 1996–present (most common on 150k+ mi vehicles)
Most likely cause on Toyota
Degraded catalytic converter or aged downstream O2 sensor
Known Toyota engine-family issues
On 2007+ California-emissions Toyotas, replacing the cat with a non-CARB-compliant aftermarket unit is illegal AND the PCM is calibrated to detect the lower precious-metal loading and throw P0420 anyway. Either replace with OE Toyota cat ($1200–$2200) or a CARB-EO-stamped aftermarket (Magnaflow CARB cats run $400–$700).
Toyota-specific causes (most common first)
- Aged downstream (post-cat) O2 sensor reading slow — Denso O2 sensors past 150k miles often respond too slowly for the cat-efficiency monitor even though the cat is fine. Replace this BEFORE the cat ($60 vs $1200).
- Genuinely degraded catalytic converter — common past 200k miles, especially on engines with prior misfires or oil-burning history (the 2AZ-FE in particular)
- Coolant leaking past a failed head gasket into the exhaust (silica/coolant contamination kills cats fast)
- Aftermarket cat (post-2007 California vehicles must have CARB-EO-stamped cats; non-EO cats throw P0420 by design due to lower precious-metal loading)
- Engine oil burning — past oil consumption from worn rings (2AZ-FE) coats the cat substrate
- Exhaust leak ahead of the rear O2 sensor causing it to sample atmospheric oxygen
Toyota-specific diagnostic tip
Before assuming the cat is bad, replace the post-cat (downstream) O2 sensor with a Denso unit and clear the code. On Toyotas, this fixes 30–40% of P0420s for $60 because the ECU's P0420 logic depends on the rear sensor responding sharply enough to confirm the cat is "smoothing" the exhaust gas — slow rear sensors trigger P0420 even on a healthy cat.
Symptoms drivers report
- Check engine light
- Slight loss of power or fuel economy
- Rotten-egg smell (severe cases)
- Emissions test fail
Typical repair cost on Toyota
Most Toyota owners fix P0420 for between $60 and $1400, depending on which underlying cause turns out to be at fault. Start with the most-likely cause for your vehicle — Degraded catalytic converter or aged downstream O2 sensor — before throwing parts at it.
Toyota P0420 FAQ
How long can I drive with P0420 on my Toyota?
Indefinitely from a drivability standpoint — the engine runs normally and you may not notice any difference in fuel economy. However: (1) you'll fail emissions in any state with OBD2-based testing, and (2) if the cat continues to break down, ceramic substrate pieces can break loose and damage the rear O2 sensor or get sucked into a turbo on Toyota turbo models.
Will P0420 hurt my Toyota's gas mileage?
Only marginally (typically 1–2 MPG worse if any) when caused by a slow rear O2 sensor. If it's caused by an actual catalyst breakdown plus an exhaust restriction, you may see 2–4 MPG worse and reduced power. Fuel trims aren't affected by P0420 alone — the rear O2 sensor is informational, not used for active fueling on most Toyotas.
Is the catalytic converter covered under Toyota's warranty?
The catalytic converter is covered under federal emissions warranty for a defined period — check your warranty booklet for current terms, since they vary by year and CARB-state status. Out of warranty: dealer-installed cat is typically $1500+ for parts and labor; a Magnaflow CARB-EO cat installed at an independent shop is usually in the $700–$1100 range. Always confirm CARB EO-stamped if your vehicle was originally California-emissions.
Related codes
Drive a different make? See the general P0420 guide for cross-vehicle causes and symptoms.
Confirm P0420 on your Toyota in 60 seconds
AXLY.pro is a free iPhone app that pairs with any Bluetooth OBD2 adapter to read P0420 along with freeze-frame data and live engine readings. No subscription.