Subaru P0420 Catalyst Efficency

Screenshot of codes

There are two DTCs present – P0325 for Knock Sensor 1 Circuit, and P0420 Catalyst Efficiency Below Threshold (Bank 1). The knock sensor DTC was a straight forward diagnosis. The pigtail for the sensor was damaged. The Catalyst Efficiency DTC is another matter. The part is very expensive and the net result to the customer of replacing it in this case would simply be turning the MIL OFF. This makes for a hard sell and increased responsibility for the technician to be correct in the diagnosis.

A/F sensor waveform

To determine catalyst efficiency, the PCM makes a comparison of the switching rate of the upstream and downstream 02 sensors. Many technicians want to verify the validity of the DTC by monitoring the 02 sensors with either a scan tool or a lab scope. The problem is that this engine utilizes a wide band Air Fuel Sensor upstream, and a conventional 02 on the downstream side of the catalyst.

Scan before A/F replaced

In the graph of scan data above, the upstream sensor appears to be switching while the downstream sensor is not changing with the same frequency or voltage fluctuation. Based on this information alone, the catalyst appears to be OK.. If the shop has a gas analyzer or an ASM dynamometer, the gasses could be used to check the catalyst but there may be a better method.The technician can use Mode $06 to see how badly the catalyst failed, if there are any outside factors affecting the catalyst, such as misfires or fuel trim.

This particular vehicle doesn’t show misfires in Mode $06,but there are some interesting observations here:

  • Note that catalyst monitoring is within specification at this time but by a small margin. This may be an indication that testing should be done under
    different operating conditions to see failure.
  • The upstream A/F sensor is also on the verge of failing.

The A/F sensor will certainly effect the Catalyst Monitor but is a very expensive component. At this point it may be helpful to perform a TSB and/or recall search. There is a recall notice (WVW-80) for the front A/F sensor. The technician informs the customer of the recall but the customer opts to have the repair done by the shop rather than take it back to the dealership. The sensor is replaced, all the codes are cleared and the vehicle is driven for a few days and the MIL remains off. The Monitors have all run, and the catalyst results are much better than they were before repairs. The customer was informed that they may still need a catalyst and a rear 02 in the future.

Scan after A/F replaced

This post was written by: Martin Hand

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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.

One Comment

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