Error Code P0421 is described as Warm Up Catalyst Efficiency Below Threshold (Bank 1). This means the Catalytic Converter’s Oxygen Sensor Downstream has detected that the Bank 1 converter is not working properly.


The catalytic converter is located at the very end of the emission diagnostic system, and there are quite a lot of things that can go wrong in this system and affect all the connected systems, particularly on the upstream of the converter.

The Error Code Error Code P0421 is actually triggered when the emissions of the Bank 1 Warm Up Catalytic Converter has gone below the minimum allowable threshold. And this code usually occurs after the engine has had numerous cold starts in the past couple of days.

However, it is important to note that, if this Error Code shows up, or other related codes such as P0420, P0430 and P0431, it doesn’t always automatically mean that the converter is in bad shape. Rather, all these codes simply mean the converter is running below the efficiency it is expected for, or it is running below threshold.

Common Symptoms

  • Check Engine light is on
  • Defective or non-functioning catalytic converter
  • Oxygen sensor malfunction (not being able to read)
  • Fouled up spark plug
  • Noticeably lack of engine power

Possible Causes

  • Faulty Warm Up Catalytic Converter
  • Engine misfire, causing damage to the Catalytic Converter

How to Check

Test drive the trouble vehicle and take note of its drivability issues that may indicate problems on the fuel or spark delivery, such as hesitation, misfires, spark knock or stumbling.

Then, idle smoothness: check for signs of roughness or surging, as they may indicate engine misfire or wrong delivery of fuel.

Inspect the tailpipe, especially following start-up. For smoke indications, white smoke means water/anti-freeze, blue smoke means oil problem, while black smoke refers to rich AFR.

Check the engine carefully and pay attention to signs of leaks in the vacuum or exhaust manifold.

Check the air filer and inspect the spark plugs. Then, scan tool for the test. Never reset the MIL light.

Ensure that the whole test have been covered when reading the OBD-II readiness test. If the test have not been completed however, chances are the MIL was recently reset, and there are some hiding intermittent problems.

Then, read both the trouble code and the pending trouble code, if necessary.

Read the scan data list when the engine runs and at its operating temperature. This is usually a table that lists all available sensors and outputs.

Check the LTFT (Long Term Fuel Trims). This will show you how much (in %) the vehicle’s computer work to balance the fuel deliver, based on oxygen sensor feedback. Positive digits may means the computer is adding fuel, while negative digit means it is removing fuel. Large positive numbers such as >10%, should be thoroughly checked, as it may mean the computer is adding a lot of fuel than it was originally designed.

Check the output signals of the oxygen sensor. Sensor 1 is located before the converter, Sensor 2 is right behind the Converter, Bank 1 and 2 in V-configuration where Bank 1 is on the side Cylinder 1 is located.  Some inline 6 cylinder engines have Bank 1 as Cylinders 1-3 and Bank 2 as Cylinders 4-6.  The sensors are usually abbreviated as O2S1B1 (O2 Sensor, Bank 1)

Sensor 1 must have attractive results, and swing quickly from around 0 to less than 1V. If the signal still shows high voltage with small fluctuation, and unburnt fuel reaching the sensor. If the sensor shows low to no voltage, this means the sensor could be defective. It can also mean there’s an exhaust leak behind the sensor. Also, if after rapid accelerator kick down, then there must be really wrong with the exhaust leak, or lack of indicated fuel.

How to Fix

The easiest way to fix for this problem is to replace the Oxygen sensor. In most cases however, it requires replacement of the whole catalytic converter.

The first thing you need to do to fix this Error Code is to measure the voltage of the Bank 1’s Oxygen sensor. This is the rear sensor, or the sensor next to the converter. As a matter of fact, it is a good idea to test each oxygen sensor while you’re at it.

The best thing to go at this however, is to fix the problem first, before replacing the Converter.

Also, when this error code shows up with other error codes, it is always better to start diagnosing and fixing the other codes first.

One thing you need to remember is that, many vehicle manufacturers offer long warranty in emission system-related components. So if you own a new car, but it is already beyond its bumper-to-bumper warranty, you may still be able to take full advantage of the warranty coverage for this kind of problem.

Many manufacturers even offers up to 5-year warranty for emissions system, some even offer unlimited mileage for these parts.

Parting Tips

For more information about different Error Codes out there, check out our website. For professional help, feel free to Contact Us.