P0420 diagnostic trouble code is a “Catalyst Efficiency Below Normal” on bank one. This applies to vehicles equipped with OBD2 only. It basically means that your computer has detected that the Catalytic converter has failed on bank one. (Bank one is the side that the number one cylinder is on.  Bank two is the other side.)

To be able to diagnose this p0420 converter efficiency code, we need to have a good understanding of how this system works. From all of my years going to class the easiest way I can describe it to you is that the catalytic converter stores oxygen. When it goes bad, it no longer has the ability to store oxygen. On a computer controlled OBD2 vehicle, there will be one oxygen sensor before the cat and one after the cat. When the engine is running and is fully warmed up, the computer compares the two o2 oxygen sensor signals for a predetermined amount of time. If the converter is storing oxygen, then the rear oxygen sensor signal will produce a much different signal back to the computer than the front oxygen sensor. This is good. This is what the computer is looking for. This means the catalytic converter is doing its job (storing oxygen).  When the computer sees that the two oxygen sensor signals are the same, this indicates that the catalytic converter is no longer storing oxygen like it should.  The computer will then light your check engine light or malfunction indicator lamp and store the fault code P0420.

How do I know if my converter is bad?   There are a number of ways to check the catalytic converter. One good way to check your converter to see if it is “lighting off”.  This means that the converter is burning up the emissions on the inside. Remember, the hotter the converter gets, the cleaner it will run. The converter should be about 100 degrees hotter going out that coming in. You can use a temp gun to check for this. You are going to want to be sure that the engine is full warmed up and run the engine at 200 rpm for about 1 to 2 minutes to prior to running this test.

Another thing that can happen is the converter can plug up and cause a false P0420 code in the computer.  This usually happens when the engine has had a misfiring or rough running in the past and the customer continues to drive the vehicle for a long period of time.  The converter will in fact get so hot that to the point that it will actually melt the brick inside into a ball.  When it cools back down, it can tend to cause a low power issue. You can use an exhaust back pressure tester to check for this condition.

Why would the computer flag this code if the converter is good? What other possible causes would there be that would cause this p0420 code in the computer?  Besides the two possible causes I mentioned above, there are several things to take into consideration.  Exhaust leaks, bad oxygen sensor or sensors, lack of power to the oxygen sensor, bad PCM (computer) or even a PCM that needs to be updated or re flashed can all cause your P0420 catalytic efficiency code.  If you have a P0420 code in the computer along with an oxygen sensor code, be sure to diagnose and fix the oxygen sensor code first before going after the p0420 code.


P0171 is a very common code for any ODB2 vehicle. I have have been dealing with this diagnostic trouble code for years on many different year make and model vehicles. Everyone always asks me is “what is the fix for this diagnostic trouble code?” Everyone thinks there is one fix for all. I hate to break it to you, but there is no “one” fix for this code. This code just simply means the engine is running “lean”. Not enough fuel for a given amount of air or too much air for a given amount of fuel. Well, let me ask you this, what can cause an engine to run lean? This is the question I have to ask myself every time I diagnose one of these codes. If you have an air or a vacuum leak, would the engine run lean? Of course it would! IF you have low fuel pressure, would the engine run lean? You bet! (see how to check fuel pressure). What if the engine had plugged fuel injectors or water in the gas? You guessed it! It would run lean!

Now, your next question should be, “how do I diagnose a lean code on my car? This my friend, is the 64 million dollar question. Just like a doctor to a sick patient, every case is different. Depending on what type of vehicle you are working with will depend on what could be wrong which will therefore dictate how you are going to go about diagnosing this code.  The cars computer determines injector pulse width based on the inputs it receives from various sensors.  Based on this, the engine will run rich or lean. Ideally, you want it to run right int he middle between the two. If one of these sensors are off or “skewed”, the computer will run the engine either slightly rich or lean depending on how the sensor goes bad.   But again, this is just one side of it. Even if all of the sensors under the hood are good, if you have an air or a vacuum leak or if the fuel pressure is too low, then this will cause the air fuel ratio to be off and cause your p0171 lean code.

The first step in diagnosing a p0171 lean code is to check for air or vacuum leaks.  This is a very important step!  I cannot even begin to tell you how many air/vacuum leaks I have come across resulting in a p0171/p0174 lean code and or causing a major idle or performance problem!  I would just pop the hood and BAM!  It would hit me right in the face it was so obvious!  However, most major vacuum leaks can be found by just by opening the hood and listening for a hissing sound while the engine is at idle as it will be pretty obvious.  However, not all vacuum leaks are easy to find.  Sometimes the intake manifold gasket or throttle body gasket will leak vacuum or even a small vacuum hose or line will leak engine vacuum making it really hard to diagnose.  If you suspect a vacuum leak causing your p0171 lean code but it is next to impossible to find, you can use a smoke machine to smoke check the intake manifold to help find the leak. A smoke machine is actually made to smoke the evaporative emissions system for evap leaks.  It works wonders!  However, my recommendation is to use this smoke machine and smoke test the intake system to check for air/vacuum leaks.  The smoke pro makes a pretty good product. It is a bit expensive, but i will save you a tone of time.  In addition, if you get one, you can make a couple of bucks smoke checking your freinds and families evap and intake systems when the time comes.

If there are no air or vacuum leaks, you are going to want to rule out low fuel pressure.  Low fuel pressure can certainly cause the engine to run lean and flag your p0171 lean code, but I do not see this one very often causing this code.   Don’t get me wrong, low fuel pressure is very common, but the customer will come in with a low power or even an no start complaint before the computer gets a chance to set a p0171 lean code in the computer because of low fuel pressure.  But if you think you have low fuel pressure, you can check out this link to read about how to check fuel pressure.

Another thing that can cause your p0171 lean code would be incorrect valve timing.  It sounds strange, but if the valve timing has jumped, then the intake valve or valves for each cylinder will not stay open as long as they should while the piston is moving down.  This will prevent the cylinder from getting all of the air in needs for the combustion process and will result is a lean condition.  Engines with timing chains will usually not jump time.  Import vehicles (including some American vehicles) with timing belts are much more likely to jump time for various reasons.  I am not going to get into this too much here because it does not happen very often.   I just wanted to bring it up so that you are are of this.  If you think this may be an issue with your vehicle, you can check the engine vacuum at idle to see if it is nice and steady.  The engine vacuum will vary depending on the vehicle you are working with.  However, just about all internal combustion engines with a four stroke engine that are street legal will produce anywhere from 18 to 22 inches of vacuum at idle.  IT is very important that the engine vacuum remain steady at idle.  If it is bouncing around or going up and down slightly as the engine is idling, then you could have a mechanic issue or even an air or a vacuum leak like we talked about earlier. If the valve timing has jumped, then the engine vacuum will be a bit lower than normal at idle. You can pick up a Vacuum Gauge at Amazon.com for only 20 bucks.

You see, there can be a few things that can cause this p0171 lean code.  We did not cover everything, but at least you get the idea of the lean condition and what can cause this code.  I hoped you learned from this article as I tried to keep it very basic.  We will cover more in depth about this p0171 lean code and other possible causes at a later date. 🙂

How to check fuel pressure

Checking fuel pressure is very vital in diagnosing an engine performance issue. If you have low fuel pressure, it can cause all sorts of engine performance problems such as a hard start, low power, poor emissions and even bad gas mileage.  If it goes on too long untreated, the computer will pick this up as a lean condition and flag a lean code such as a P0171.  Depending on the application, it will take any where from a few moments to a few minutes to hook up a fuel pressure gauge.  Most fuel rails have a test port that you can screw the fuel pressure tester into in order to get a fuel pressure reading.  Others you will have to tap into the fuel line by disconnecting the fuel pressure line from either the fuel rail or the fuel filter and installing a fuel pressure gauge in between the two in order to check fuel pressure.

Some fuel systems are only at 30 to 40 psi while others range in the 50 to 60 psi range or more.  Once you know what your fuel pressure is at, you are going to want to get the correct specifications for your vehicle to see if your fuel pressure falls within the factory specs.  You can go to AllData Do It Yourself or another source to look up your specifications for your vehicle.  If the fuel pressure is low, be sure that the fuel filter and the fuel pressure line are not pinched or plugged up in any way and that the fuel pump has full battery voltage before you replace the fuel pump.

There are several different type of fuel pressure testers.  How do you know which one is right to use on your vehicle when testing fuel pressure?  The answer is as long as you have a “normal” vehicle, you should be able to get away using any type of fuel pressure gauge.  What do I mean when I say normal?  Any modern-day vehicle.  Old carburetor or hot rod vehicles are not going to apply here.  Just about any 4, 6 or 8 cylinder fuel injected vehicles (even some 5 cylinder engines) are the ones we are talking about.

If you want a good quality fuel pressure kit that will do just about everything and will last for a life time, I would recommend to go with an OTC or a Snap On fuel pressure master kit.  The OTC Master Fuel pressure Kit will run you about 187 bucks from Amazon.com.   It is a much better price than Snap On’s master kit which last time I checked it ran on upwards about $500 bucks or more.

A cheaper alternative would be one of the Actron fuel pressure gauges.  They seem to work ok as far as measuring the fuel pressure and being accurate, but have their quirks.  The seals and o rings need to be replaced regularly in order to prevent them from leaking fuel.  So just careful.  The good news is that it is cheap, simple and easy to use.  The Actron CP7838 Fuel Pressure Tester is a good choice as it comes with the basic adapters.  You can pick this up for about $44.

Another good way to check to see if your fuel pump is bad is to check to see how many amps it is pulling.   You will need either a voltmeter that has an amp setting, a voltmeter that works with an amp clamp or a lab scope.  I use a Fluke 87 with an amp clamp for this job.  It is a fantastic and extremely reliable voltmeter.  (Don’t quote me, but I believe this meter is so accurate that it is certified to work on aviation/air crafts).  You can either clamp around the power wire going to the fuel pump or you can put a jumper in place of the fuel pump fuse and clamp around the jumper wire to get an amp reading with your voltmeter.  You need to check this while the engine is cranking over or when the fuel pump is running.  On some vehicles, you can remove the fuel pump relay and jumper the hot wire going to the fuel pump relay (with the key on) to the fuel pump feed wire going to or leaving the fuel pump relay going to the fuel pump and the fuel pump should run.  (Each vehicle is different so be sure to consult with a wiring diagram for your specific vehicle.)  The amount of amps your fuel pump will pull will depend on how much fuel pressure your fuel pump should supply to your fuel rail.  A good rule of rule of thumb for this test is as follows:  14 to 15 psi 3 to 4 amps, 30 to 45 psi 4 to 7 amps, 45 to 50 psi 7 to 8 amps, 50 to 70 psi 8 to 12 amps

Happy fuel pressure testing!