did autolite spark plugs screw up coil or coils ?? - Toyota Yaris Forums

YarisSedan · 04-06-2009, 12:30 AM

Okay ill try to help you best i can. P0420 id say 75 percent of the time this code is on for any car that comes into my shop it means the catalitic converter is bad. other 25 percent it is a o2 sensor. If you want to try to diagnose this yourself you would need a scanner even a cheap generic scanner that shows live data. You should be able to see o2 sensor voltage before the cat and o2 sensor voltage behind the cat. If the voltage is the same than more than likely the cat is to blame.

If you want to test the o2 response you can introduce fuel into the system and see if the o2 responds by spiking the voltage. You can do this with propane gas entering the intake you should see the mixture go rich and the fuel trim start to max out. Or if not available you can try carb cleaner.

If the o2s respond properly then the cat is the problem.

Anotherway to test the cat is purchase a pyrometer. It shoots a beam and measures the temperature of whatever it is aimed upon. Measure the temperature before the cat and after. The exhaust gas as it enters the cat gets heated up to burn off any unburned fuel in the combustion chamber. Usually the temperature is 100 degrees hotter exiting the cat. If the temperature is the same both inlet and outlet or is even cooler in the outlet then you know for sure the cat is bad. A cheapo pyrometer could run you around 40-50 bucks same as a generic obd2 scanner.

If you find out the case the cat is bad which it probally is after thourouly reading your post then i would say take it to a muffler shop and make sur eyou get a OBD2 CALIFORNIA APPROVED catalitic converter. I would try replacing the main cat first and leaving the precat alone. Then clearing the code and see if the light stays off.

After reading your othe rposts. I dont know why your mechanic would suggest a coil. Unless your car is missfiring with a blinking check engine light. You have no coil problems. Plus any decent mechanic should know how to test coil winding resistance to verifiy if it needs to be replaced or not.

What i would reccomend is putting a can of seafoam in the gas tank. As well as running a can through a vaccum line through the intake. It has cleaning properties which will clean the tips of the o2 sensors as well as clean the cat of debri.

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04-06-2009, 12:30 AM	#12
YarisSedan Drives: 08 Yaris Sedan Join Date: Oct 2008 Location: California - Bay Area Posts: 2,773	Okay ill try to help you best i can. P0420 id say 75 percent of the time this code is on for any car that comes into my shop it means the catalitic converter is bad. other 25 percent it is a o2 sensor. If you want to try to diagnose this yourself you would need a scanner even a cheap generic scanner that shows live data. You should be able to see o2 sensor voltage before the cat and o2 sensor voltage behind the cat. If the voltage is the same than more than likely the cat is to blame. If you want to test the o2 response you can introduce fuel into the system and see if the o2 responds by spiking the voltage. You can do this with propane gas entering the intake you should see the mixture go rich and the fuel trim start to max out. Or if not available you can try carb cleaner. If the o2s respond properly then the cat is the problem. Anotherway to test the cat is purchase a pyrometer. It shoots a beam and measures the temperature of whatever it is aimed upon. Measure the temperature before the cat and after. The exhaust gas as it enters the cat gets heated up to burn off any unburned fuel in the combustion chamber. Usually the temperature is 100 degrees hotter exiting the cat. If the temperature is the same both inlet and outlet or is even cooler in the outlet then you know for sure the cat is bad. A cheapo pyrometer could run you around 40-50 bucks same as a generic obd2 scanner. If you find out the case the cat is bad which it probally is after thourouly reading your post then i would say take it to a muffler shop and make sur eyou get a OBD2 CALIFORNIA APPROVED catalitic converter. I would try replacing the main cat first and leaving the precat alone. Then clearing the code and see if the light stays off. After reading your othe rposts. I dont know why your mechanic would suggest a coil. Unless your car is missfiring with a blinking check engine light. You have no coil problems. Plus any decent mechanic should know how to test coil winding resistance to verifiy if it needs to be replaced or not. What i would reccomend is putting a can of seafoam in the gas tank. As well as running a can through a vaccum line through the intake. It has cleaning properties which will clean the tips of the o2 sensors as well as clean the cat of debri.