Puma 1.7 P0420 fault code - Looking for advice!


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Major Malfunction

New member
Sep 21, 2015
Hi all,
Relatively new member looking for a second opinion/advice on my Puma Thunder!

Last week my engine warning light illuminated, after connecting my OBD2 connector and using ForScan it highlighted the P0420 - Catalyst System Low Efficiency code. I've noticed that it smells quite rich, however other than that the car drives well, with no detectable mis-fire... (Saying that I have noticed a small crack in the coil-pack body and will be replacing that anyway).

This morning I decided to monitor some live readings from the oxygen sensors O2S11 (upstream) and O2S12 (downstream), and the engine coolant temp sensor. Whilst keeping the engine speed at 2000rpm this created the following readings...

My question is, is my catalyst shot or could it be false readings due to something else? I'd really like to avoid buying a new cat! I've had a quick look for exhaust leaks, re-gapped the spark plugs, even filling it with v-power in the hope the light will go out!!

Does anyone have any suggestions for what else to look for?
Thanks in advance!
It looks like you've looked at all the right things especially checking for exhaust leaks as they would throw the lambdas out. If the car has been running rich for some time the cat may already be knackered. If you have a local friendly MOT station see if they'll put it on the gas tester for you as then you'll get a check on CO (cat) Hydrocarbons {engine) and Lambda (mixture) but your exhaust has to be completely gas tight right to the end where the probe goes in, oh and make sure it's nice and hot when you go in.
Small update (kind of)...

Booked it into the garage, only for the light to go out that morning....typical! Anyway they measured the emissions which it passed so I know the cat is OK, they also connected their OBD2 and found an additional code, P0136 (Ox Sensor Circuit Malfunction (Bank 1 Sensor 2)) which must have cropped up recently. The garage didn't go any further with diagnosis as they gave some rubbish about not being able to view live sensors readings on older cars.

As emissions was OK and the light was out, I drove it around for a few weeks. Until today when the light reappeared :evil:

A quick check revealed P0136 (but no P0420). Also did another live oscilloscope reading but this time it looked a little different. After a couple of minutes the downstream sensor just drops out to around 0.01V - 0.02V....hmm that doesnt look right!
OxySensTest5.png Well as a new sensor from eurocarparts is only £25 with a discount I might as well stick a new one on...wish me luck!

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