P0420 Diagnosis

P0420 Diagnosis

 

The P0420 is commonly refered to as the Cat Code, because replacement of the Catalytic Converter is usually the repair assosiated with this Code. However a few other things must be checked also.

 

This is a picture of a common Catalytic Converter.

 

 

 

This is a picture of a Manifold Converter, notice how it is built into the Exhaust Manifold as a unit. These are obviously more expensive than a normal Converter, require more labor to put in and are commonly found on newer inport Vehicles. Many cars are going in this direction.

 

Im not the type of person who likes to guess at things, i like to have the proper testing equipment to verify what is going on. However i understand that not everyone is going to have these tools. So if you are looking for the most likely option to get rid of this pesky P0420 code, i would first try to verify there is no holes in the Exhaust around the Catalytic Converters / 02 Sensors. If none exist i would replace the Catalytic Converter. Read on for more technical details on this issue including how to see a failed converter on a scan tool.

 

Below is a rather large hole in the Exhaust Pipe

 

 

 

 

If you have a scan tool view the O2 Sensors Data and watch the signals of the Front 02 Sensor and the Rear 02 Sensor for the effected Cat. You should notice that the Front 02 sensor will vary from Lean to Rich. Lean meaning an Air/Fuel Mixture of Mostly Air and Rich meaning a mixture of mostly Fuel.  Fully Lean is represented by a Voltage of 0.1V and fully Rich is 0.9V. Your front 02 Sensor signal should switch back and forth between Lean and Rich rapidly, the rear 02 Sensor when healthy should maintain a steady voltage around .7 Volts or so commonly.

 

Something else you want to check for is an exhaust leak before the Rear 02 Sensor. If extra air is entering the exhaust it will bring that Rear 02 Signal down below .45 when it should be hovering above this number usually. The P0420 Code gets triggered when the computer notices the Rear 02 Sensor Signal starts to look like the Front 02 Sensor. This indicates the Converter has lost its ability to store oxygen and cannot properly treat the Exhaust Gases coming through its Catalyst.

 



P0128 Diagnosis

P0128 Diagnosis

 

MY P0128 Video is below this Article

 

 

The P0128 Code reads as "Thermostat Below Regulating Threshold". The P0128 Code is actually really simple to diagnose. It is usually a Thermostat sticking open that will cause the issue, however we need to also check the Coolant Temp Sensor (ECT, CTS) also before we condeme the Thermostat.

 

Let me save some of you some time. Like i said this is usually always a Thermostat stuck open causing this. If you dont like to waste alot of time and you have not had a new thermostat in a while, just go ahead and do the Thermostat. Thermostats are Cheap! ($10 with a seal/gasket) Otherwise you can read more into my blog on how i describe the operation of the thermostat and in a little more detail the reasoning behind this code being set and some things to check with your temp sensor.

 

The Coolant System is designed to block off the flow of coolant thru the radiator until the car has reached a predetermined temperature (This is what the Temp rating on your Thermostat means). When the car does not reach this temperature in a set amount of time and the car determines this by use of other sensors that the car has been running long enought that it should have reached proper temperature, it will Illuminate Your Check Engine Light and Set Trouble Code P0128. It will do this not the 1st time is sees the problem, but the 2nd time the car is started and driven and it sees it again. This is called 2 Trip Logic.

 

Watch your temp guage on your dash, is it taking longer than usual to reach the middle of the guage? The best way to check the Coolant temp sensor is to have a Scan tool Device that will read Data, and make sure the Coolant temp is moving. If the Coolant Sensor had failed all the way, it usually resorts to a reading of -40 Degrees(Yes thats Negative 40 Degrees) and will set its own code (P0118) and the P0128 Code will not illuminate with a Coolant Temp of -40 Degrees because it knows already its the Coolant Sensor. Is the guage even going all the way to the middle? Usually with this code the guage will not even make it all the way to the middle. You can CAREFULLY touch your raditor hoses very quickly to feel if they are up to temp or not, if they still feel cool after the car has been running more than 5-10 minutes you can almost bet your Thermostat is sticking open. This might not seem like a big deal to most people. "Well at least its not stuck closed, and causing it to overheat"! But guess what ITS WASTING YOUR GAS!! For every second the car has not reached Proper Operating temperature Your Computer is demanding that extra fuel be added to the mix to help the car warm up faster! Not to mention your oil does not come up to temperature like its accustomed to so it will overtime begin to "Sludge"...think of that word..."Sludge"...it just doesnt sound good and we know its not good. So dont let this go too long!!

 

Questions? Email me at MyPeoplesGarage@gmail.com

 

My P0128 Diagnosis Video Below!!!