Last update:


  OBD2 - Bad Cats

About Us Useful Links Forums Mailing List


Easy Guide to Scanning
OBD2 - Fuel Pressure
OBD2 - Faulty HO2S
OBD2 - Bad Cats
OBD2 - Lambda
OBD2 - Bad MAF
OBD2 - Poor running
OBD2 - Poor Running TPS
OBD2 - Refresh Rate


OBD2 Fault Codes -

Good CAT, Bad CAT

A good example of the Catalyst Monitor on OBD II is shown on this scan from Mike W.

Mike had just had the nearside catalyst replaced on his 24V Cosworth and with the latest scan he wondered if there was something wrong with it, but in fact his scan was the perfect example of one good and one failing catalyst all on the same graph. Below is a sample of the graph from Mike's scan.

The top plot shows the reading from Bank 1 (drivers side of the engine, cylinders 1,2 and 3.) The red line is always the upstream sensor (HO2S11) and the blue trace is the reading from the downstream post cat sensor, HO2S12. The upstream sensor shows switching from a high to low reading , but so does the post-catalyst sensor. If the catalyst were working properly, the reading from the post cat sensor would be an almost steady line, with a very slow switching rate.

And, on the lower plot, the good catalyst. This shows clearly as the one which has been replaced. The upstream sensor HO2S21 shows much the same switching here as on Bank 1 (brown trace) but the grey line (HO2S22) shows the steady, slow switching of a healthy catalyst.

There is no telling how long the Bank 1 catalyst will last before it starts to generate DTCs (Diagnostic Trouble Codes.) When the ratio of Upstream to Downstream sensor switching approaches 75% then the Catalyst Monitor will store the long-term DTC P4020 Catalyst Efficiency Below Threshold Bank 1. On later EOBD equipped cars the Malfunction Indicator Light (MIL) would illuminate, but although the EECV is capable of switching on this light, it was not installed on the Scorpio. For us, there is no evidence of this, except that CO and HC levels will be high enough to fail the emission check on the next MOT.

Thanks to Mike for the Scan




Copyright 2003