What Does P0420 Mean?
This code is commonly known as 'Catalyst System Efficiency Below Threshold (Bank 1),' though the description in this batch references the AIR relay B. In standard OBD-II, P0420 indicates that the bank 1 catalytic converter is not reducing exhaust pollutants efficiently enough. The rear (post-cat) oxygen sensor waveform is mirroring the front sensor, meaning the catalyst substrate is no longer storing and releasing oxygen properly. The driver may notice no driveability symptoms but the MIL will be on.
Common Causes
40%
Deteriorated or contaminated catalytic converter substrate (age, oil/coolant fouling)
20%
Failing rear oxygen sensor (bank 1 sensor 2) providing inaccurate efficiency readings
15%
Exhaust leak upstream of the rear O2 sensor skewing readings
15%
Engine running rich or lean due to fuel system issues degrading the catalyst over time
10%
Engine misfire causing raw fuel to overheat and damage catalyst material
Diagnostic Steps
1
Monitor both bank 1 O2 sensors simultaneously on the scan tool. The upstream sensor should show active switching (0.1-0.9V). The downstream sensor should be relatively steady (0.5-0.7V) if the cat is healthy. If the downstream closely mirrors the upstream, the catalyst is failing.
2
Check for exhaust leaks from the exhaust manifold to the rear O2 sensor location using a smoke machine or by listening for hissing with the engine running.
3
Inspect freeze frame data for the fuel trim values when the code set. Long-term fuel trims beyond +/- 10% indicate a fuel delivery issue that may be causing or contributing to catalyst degradation.
4
If possible, use an infrared temperature gun to measure the catalytic converter inlet vs. outlet temperature. The outlet should be 50-100°F hotter than the inlet if the converter is functioning. Equal or lower outlet temps indicate a dead catalyst.
5
Before condemning the converter, verify no active misfires or fuel system codes. Fix any underlying issues first, clear the code, and retest — sometimes a fresh converter fails again quickly if the root cause is unaddressed.
6
Check the rear O2 sensor response by temporarily enriching the mixture (propane test). The downstream sensor should respond, but more slowly and with less amplitude than the upstream sensor.
Estimated Repair Cost
$200 - $2,500
Parts + labor, varies by vehicle and location
This code is commonly known as 'Catalyst System Efficiency Below Threshold (Bank 1),' though the description in this batch references the AIR relay B. In standard OBD-II, P0420 indicates that the bank 1 catalytic converter is not reducing exhaust pollutants efficiently enough. The rear (post-cat) ox...
The most common cause of P0420 (Secondary Air Injection System Relay "B" Circuit Malfunction) is: Deteriorated or contaminated catalytic converter substrate (age, oil/coolant fouling)
Typical repair costs for P0420 range from $200 to $2,500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to continue driving in most cases. The vehicle will produce higher emissions and will fail emissions testing. If the catalyst is severely degraded, it may overheat under sustained load, which is a rare but potential fire risk. Address other engine issues first to prevent further catalyst damage.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0420 to identify the root cause.
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Quick Info
Category
Powertrain
System
Catalytic Converter / Exhaust System
Difficulty
Type
Generic (SAE)
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