Hi just had cruise/anti skid/engine lights come on plugged in code reader and it`s code p0420 i see this is very common code to come up whats the most likely cause.

Edited June 20, 201510 yr by nickb

what year is your car?

Anything that affects the air/fuel ratio, or the burning of the air/fuel mixture in the engine will cause a P0420 code. Any exhaust leaks between the engine and the rear-most oxygen sensor can also cause the code. Poor battery and ground connections have also caused this code.

 

The best place to start is usually with a good tune-up if it hasn't had one recently.

Anything that affects the air/fuel ratio, or the burning of the air/fuel mixture in the engine will cause a P0420 code. Any exhaust leaks between the engine and the rear-most oxygen sensor can also cause the code. Poor battery and ground connections have also caused this code.

 

The best place to start is usually with a good tune-up if it hasn't had one recently.

I have read even an old clogged air filter can cause it.

what year is your car?

it,s a 2003

I have read even an old clogged air filter can cause it.

thanks will start with the air filter

Anything that affects the air/fuel ratio, or the burning of the air/fuel mixture in the engine will cause a P0420 code. Any exhaust leaks between the engine and the rear-most oxygen sensor can also cause the code. Poor battery and ground connections have also caused this code.

The best place to start is usually with a good tune-up if it hasn't had one recently.

it has just had a new stainless steel exhaust put on

it has just had a new stainless steel exhaust put on

Is that a freer flowing exhaust?  Even a reduction of back pressure might cause it. I got that code after removing the rear cat.

Edited June 20, 201510 yr by desertsubaru

it has just had a new stainless steel exhaust put on

Can you be specific on which part of the exhaust?

Can you be specific on which part of the exhaust?

as far as I know it had a complete exhaust from cats back

Is that a freer flowing exhaust?  Even a reduction of back pressure might cause it. I got that code after removing the rear cat.

it,s certainly a larger diameter exhaust,it had just been done when I bought her

The rear O2 (downstream) could have been disturbed or there's a small leak somewhere between the converters. Or, there is the possibility that if the car had been running poorly (or a CEL for a non-emissions related issue like a knock sensor or upstream sensor that had been neglected for a long time), it's possible to have plugged or damaged the converters.

 

A decent code scanner can watch/graph the rear O2 voltage. If it changes with the upstream voltage, it's usually good. If not, could be bad, dirty or a bad converter.

A decent code scanner can watch/graph the rear O2 voltage. If it changes with the upstream voltage, it's usually good. If not, could be bad, dirty or a bad converter.

Rear O2 sensor voltage should be fairly steady around 0.45 v. It can vary, depending on engine load, but if it follows the front sensor voltage exactly that shows that the converter is not working.

 

 

There are other things to look at on a scanner to determine if the converter is actually bad or if its not working because the AF ratio is too far out to allow proper operation of the catalyst.

Not sure (other members please confirm) but I read somewhere that if you use a temperature gun and scan the front joint and the rear joint of a catalytic converter that the rear should be approximately 70-80 degrees hotter.

Let the car run for 20-30 minutes. Then use your temp gun, the back should
be at least 75 degrees hotter than the front. If the front is hotter
than the back, then the cat is restricted. As far as a range (once car
is warmed up) something like 275 front and 375-400 back would be
acceptable. If its under 250, the car isn't warmed up enough and if it
temps something stupid like 550 front and 650 rear, then there is a
problem.

Let the car run for 20-30 minutes. Then use your temp gun, the back should

be at least 75 degrees hotter than the front. If the front is hotter

than the back, then the cat is restricted. As far as a range (once car

is warmed up) something like 275 front and 375-400 back would be

acceptable. If its under 250, the car isn't warmed up enough and if it

temps something stupid like 550 front and 650 rear, then there is a

problem.

Catalytic converters don't work at all until they're above 500°F. By about 600° a cat is only working at about 50% capacity. Most cats need to be around 900°F to be fully operational. Some newer designs need to be around 1,400 to 1,600°F.

as far as I know it had a complete exhaust from cats back

If it's just a cat-back system that's only half of the system, and it's the half that doesn't really matter.

 

If the header/y-pipe and cats were also replaced then they could be the cause for the code. Aftermarket y-pipes don't have heat sheilding which means the exhaust gas loses more heat before it gets to the converters, them there isn't enough heat left to get the converter up to working temp. Wrapping the y-pipe with header wrap will fix that.

Catalytic converters don't work at all until they're above 500°F. By about 600° a cat is only working at about 50% capacity. Most cats need to be around 900°F to be fully operational. Some newer designs need to be around 1,400 to 1,600°F.

It also depends on the size of the cat. Walkers site says 500f. Either way the digital thermometer can be used to check for an emissions problem.

http://www.walkerexhaust.com/support/diagnosing-converter-issues/check-converter-temperature