Tech Tips

How to Deal with PO420 Codes

When it comes to service issues on today’s vehicles, the difficulty in correctly diagnosing and evaluating catalytic converter problems has to be at top of the list. One of the most annoying is the persistent PO420 diagnostic code (Catalyst System Efficiency Below Threshold (Bank 1), which not only shows its teeth when the catalytic converter is on the blink, but can also be generated by a variety of engine problems not directly related to the converter.

Possible PO420 code causes

The following engine related problems are known to generate the PO420 code:

  • Intake manifold air leaks
  • Fuel injector problems (leaks)
  • Incorrect spark plugs
  • Ignition timing
  • EGR problem
  • Defective catalytic converter
  • Oil or antifreeze entering exhaust
  • O2 sensor not operating correctly
  • Road damage to converter
  • Silicone contamination

Most, if not all, catalytic converter failures are caused by a problem or malfunction somewhere in the emission system ahead of the converter. So, it’s very important to determine what actually caused the converter to fail, so that the problem can be repaired and a recurrence can be prevented.

Here are some troubleshooting suggestions:

Testing the converter:

Method 1: Vacuum test

Connect a vacuum gauge to the vacuum on the intake manifold, carburetor, or throttle body. Note the reading at idle. Then raise and hold engine speed at 3,000 RPM. The needle will drop when you first open the throttle, but should then rise and level off. If the vacuum reading starts to drop, pressure may be backing up in the exhaust system indicating a blockage somewhere in the exhaust system.

Method 2: Backpressure test

Measure backpressure directly. If vehicle’s engine has air injection, disconnect the check valve from the distribution manifold and connect a low pressure gauge. Or, remove the oxygen sensor and take your reading at its port in the manifold or head pipe. A reading of more than 1.25 PSI at idle or more than 3 PSI at 2,000 RPM tells you there’s an exhaust restriction.

Method 3: Temperature test

In late model engines with fuel injection, the combustion is so efficient that the converter has little to process and the difference between the inlet and outlet temperatures may only be 50 ºF at 2,500 RPM. This is a lot less than the old rule of thumb that says a good converter should show at least a 100 ºF difference. At idle, the converter in many late-model vehicles may cool down so much that there’s almost no measurable difference between the front and back temperatures. So checking exhaust temperatures front and back of the converter at idle and 2,500 RPM may not be an accurate way to determine if the converter is working or not.

Watch out for RVT silicone contamination

Silicone-based products or Teflon sealants should not be used on any part of the exhaust system. They are not designed to operate at high exhaust temperatures and will out gas, causing damage to O2 sensors. Below are some examples of O2 sensors contamination caused by the use of RTV silicone on exhaust manifold flanges and other components as well as Teflon-type sealants used on O2 sensors.

Typical contamination problems: