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Chevrolet trouble code

I have a 96 chevy lumina sedan,recently the code for catalytic converter came up. ( I have a small code reader). From what I’ve been able to discover, this doesn’t necessarily mean the converter is bad. I’ve decided to take the car to the dealer to pinpoint the problem on their expensive code reading computer. Does that sound the right thing to do? I’m an ok week end mechanic but it seems there are too many possibilities to nail down the real cause.The code was p240 or p420 (I’m sorry I can’t remember exactly) Thanks

That would be P0420.

If you take it to the dealership, their code reader will come up with…P0420.

Don’t waste your time and money going to the dealership. If an inspection of the oxygen sensor wiring and connectors doesn’t turn up anything obvious, take it to an independent shop. Or just take a gamble and replace the downstream oxygen sensor yourself. Have you tried clearing the code to see if it returns?

What questions should I ask at an independent garage to make sure they know what to test for?

PO420 means ‘Catalyst system efficiency below threshold’.

Doesn’t that mean the post-cat O2 sensor signaled the computer that the cat converter isn’t capable of doing its job?

Why replace the O2 sensor?

I’d be inclined to test the O2 sensor first before replacing it. Just make certain the engine/exhaust system is up to operating temp first or you’ll get an incorrect test reading.`

OOPS! I re-read your post NYBo, and see you did note the sensor should be inspected. Sorry.

My husband made an appt at a dealership…before we got your reply…why should we think again about Not going there? I’m told they have more/better tests/tools?.. Or, do they just go for worst-case scenarios? Your advice defintely helped us in the past…If we go to an independent garage, should we ask about a particular test that we need? and should we drive the car for a time before the test? (Temperature considerations?) Thanks…

if the downstream sensor is failing, and is giving erroneous readings how would the same sensor know if in fact the cat was “below threshold”?

test the sensor sure. but for the cost of a cat, i would go for the cheaper idea first. actually the first sensor may be related, not knowing what the input sensor is reading, may screw up the output sensor too. (in all likely hood both sensors and the cat need replacing, but start small i say)

the reference to the dealership can be summed up by the estimate you are about to get from them:

likely it will ‘recommend’
(get the point. the KEY word here is recommend, versus required. note the word recommend. they are very careful about selling this service, and they make it ‘your choice’, but they recommend SO much extra stuff it drives the price WAY up.)

a new catalytic converter,
new o2 sensors,
new muffler, (and exhaust),
radiator flush, transmission flush, oil change, wheel bearing lube, brake job, front end alignment, interior detailing, 360 point safety check, new serpentine belt, battery service(?), brake fluid flush etc. etc. etc.

all you are really required to do are the first two, maybe either the first or the second, but the ‘laundry list’ of ‘recommended’
services will far exceed what you really need by going to a a dealer.

a small local mechanic will be more honest, and once you develop a trust with them, you can actually get things done cheaper, faster and with less BS the dealer gives.

try this link enter your zip code and look for a mechanic near you. ask among friends and neighbors, co workers and even strangers.