MIL or Check Engine Lamp Testing pt.1

The Check Engine Light is on. Why? Lets start at the beginning. Your Check Engine Light, Malfunction Indicator Lamp or whatever name yours goes by currently is actually just a small bulb in the instrument panel. We are going to call it the MIL. When your car is running positive voltage is at the MIL feed circuit. I've made an extremely sophisticated mock-up to refer to.

Depending on make, model and design, the MIL will come on when you turn on the key and maybe stay on a few seconds when first started. Those are bulb tests so you can see that the bulb does work. If the MIL stays on while running or comes on while driving there is a problem. It could be a short on the ground circuit but that is very very rare. It will usually be because the computer has detected a problem. The computer will either ground the circuit directly, or request another computer to ground the circuit, again depending on the particular system. Next then is to determine if the computer sees a reason to turn on the MIL and what that reason might be. This is when a code reader comes into the picture. When the computer determines there is a problem it is not only supposed to turn on the MIL but it is also supposed to store a trouble code related to the problem. 

There are basically two diagnostic systems built in to the MIL systems. There is a federal government mandated OBDII compliant and an OEM (manufacturer) system. OBDII readers are very common and usually not expensive. When you go to a parts store and get codes read, it is usually an OBDII reader. More expensive readers and scan tools can usually read both OEM and OBDII. It depends. There are a lot of diagnostic systems out there and a lot of testers. Over the years I've used and purchased several. So let me introduce you to my little friends. 

I have an AutoXray EZ-Scan 6000 that I've had for years. It does OEM checks on old OBDI systems up to 1995. Rare usage these days for that but it does still work as an OBDII generic reader. Here is a 2002 Taurus with MIL on and the stored code. I'll get into freeze frame and data later. For now, generic OBDII codes.

AutoXRay EZ-Scan 6000

An old OTC Genisys 2.0. Just using OBDII generic here as well. 

OTC EVO Genisys 5.0 & 4.0

Snap-on Solus Pro, Ultra and Edge

OTC Encore

Actron AutoScanner Plus (older and later models)

NGS MachII

OTC Monitor 4000Enhanced

Innova 3110

Alldata Mobile (tablet)

Actron U-Scan

You might have noticed all the codes were pulled from the same vehicle using each of these code readers. You might be wondering why I would have 15 different tools to check for codes. I don't really. I have 18, but was only using the ones that would pull generic OBDII codes from this particular car. Some do more tasks, some do less. Some are very expensive, some are much less so. Some are latest updates, some are better for older cars. Some are for quick basic code reads, some for more extensive testing. Remember that the cars have more than generic OBDII codes and systems.
An automobile manufacturer tool is fully capable of doing many many sorts of tests, commands and updates for a particular make of car but in the general repair shop and aftermarket we usually see many different automobiles and to be loss costly it makes sense to go with aftermarket tools.
This OTC Encore can run Ford OEM self tests.

There are also tests that may not be available on one aftermarket tool, yet present on another. You have to use the strengths of each available tool to help in the diagnostics. When you are using a generic code reader you should be aware that you may not be seeing all the stored codes. There may be OEM system codes that don't show on the OBDII reader. Good to remember if you have an MIL on but show no codes on your reader. A look at OEM codes may reveal stored codes. Also, there are many "sources" that make it seem as though all you need is codes read and immediately know what is wrong with your car. That isn't true at all. When I try to explain there is no machine that tells you what is wrong I don't always feel like I am believed. I think often the person I'm talking to just figures I must not have one. There is a LOT of false information out there so I can understand it being confusing.
So just how useful is this P0136 OBDII code? It gives a starting point. The brief code definition given by the reader says O2 Sensor Circuit Bank 1 Sensor 2. An in-line engine design has all cylinders in a single row on the engine block. As far as "bank" there would only be one. On this Taurus 3.0 we grabbed codes from though it is a V-type block. There are two banks of cylinders. Bank 1 would be the side with cylinder #1. Bank 2 would be the other side.

Bank 1 on the Taurus then is the bank nearest the firewall. The code is for bank 1 and sensor 2. The sensors are numbered as they come from the engine exhaust manifold. Bank 1 sensor 1 would be the first O2 sensor in the exhaust of bank 1. Bank 1 sensor 2 would be the second sensor in that banks exhaust. Sensors in front of the catalytic converter are usually for fuel control and sensors after the cat are cat monitor sensors.  

P0136 then indicates a problem with the catalyst monitor O2 on bank 1. B1S2. Since the post converter sensor isn't for fuel control there will probably not be a performance problem. The post converter sensor lets the computer "see" how well the converter is working, or how poorly. When the cats getting warmed up and are doing their job you can see it by watching sensor activity. Fuel control sensors should be switching well from lean to rich at full range. Monitor sensors, not so much. Sort of a slow and less active version of the fuel controls because it is after the exhaust has gone through the converter. 

This is from a 99 Chevy P/U. Both banks and both upstream and downstream sensors. You can see the difference. 

Another from a Jeep that was in recently. You can see that the bank 1 cat is not working at all. Sensor 1 and sensor 2 on that bank are mirror images. The bank 2 cat is bad as well but working a bit. When you see a monitor running like the one in b1s2 here you will get a P0420 for poor cat efficiency. 

This one is from the same Ford we captured the code from. P0136 B1S2 Circuit Problem. 

To figure this MIL problem out we have to find out what actually sets a P0136. The code would indicate the sensor is staying near 0 volts. Ford is pretty vague on the details. But we can see in the data waveform the sensor is actually near 0 volts. 

At the most basic, if all your tool does is read codes you would need to inspect the wiring, sensor and exhaust relative to the b1s2 sensor and then monitor the signal with a meter. If the wiring, etc is good then watch the signal while feeding propane to the intake. The richer fuel should make the sensor react. If it doesn't and all else was good then you do have a bad sensor. None of that is easy or simple. Point being, a code doesn't tell you what is needed to fix. The code, when it isn't misleading, is about what needs to be tested. 

I'd like to continue this as a series. I hope it is coherent. 

To start to find out why your MIL is on you will need to check for stored trouble codes.

There are OBDII self diagnostics and there are OEM self diagnostics.

You can start with a basic OBDII code reader.

You may need more to do useful testing.

I think the next level up from OBDII code reading only would be OBDII freeze frame data. Hope to cover that next. 

Thanks for reading!

Kenneth Hayes

G&G Auto Repair