FAQ-18
Check Engine Light (CEL)
or Service Engine Soon Light (SES)
This is an orange or yellow light in the dash area that is the on board computers way of talking to you, the driver and the technician. Dash lights mimic traffic lights in color. A red light means STOP RIGHT NOW! A yellow or orange light means caution, you can continue to drive, but pay attention to smells, vibrations and to your gauges.
It is also very important to know what occurred just before the light came on, if possible. For instance, a customer once told me her Suburban SES light would only come on while she was climbing a hill and directly afterwards, it began to ping real bad. You have no idea how much diagnostic time she saved herself by telling me that. I knew that it was a "load" created problem, I knew that the computer was having a tough time controlling the timing and I knew the computer had given up trying to control the timing (pinged, then quit, then the SES light came on) and set the SES light or tuned it on. I scanned the computer, pulled a knock sensor code, went directly to the knock sensor (on the engine block above the starter) and found it had been left disconnected by the shop who had just replaced the starter. I cleared the codes, drove the Suburban real hard and the code didn’t reset, Suburban Fixed!
Here’s a few scenarios that will cause the SES light to come on:
1. The computer doesn’t see the oxygen sensor or 02 sensor moving from lean to rich as it should. The 02 sensor seems to be stuck on lean so the computer sets the SES light and related code. The first thing the tech has to do is determine if the 02 which is stuck lean or rich (depending on the code) is lying, dead or telling the truth. The tech will plug in his scanner to read or pull the code and then test the ability of the 02 to see "lean" and see "rich". By creating a big vacuum leak and watching the voltage on his scanner, the tech wants to see .1v to .4v which is the lean signal he wants to see from the 02. Then by adding carb cleaner or propane to the running engine, the tech wants to see the 02 go to .7v to .9v which is the rich signal he wants to see. If the 02 is stuck lean and won’t go rich, it’s bad and needs to be replaced. If the 02 works fine, then the tech knows the engine is running lean and the 02 is telling the truth. At that point the tech will ask himself, what can cause the engine to run lean all the time. The answer, in this case, is plugged fuel injectors, plugged fuel filter, weak fuel pump, bad map or air flow, big vacuum leak or bad TPS.
2. As your are driving down the road, the computer is watching your throttle pressure, your engine speed, your vehicle speed in mph, what gear you are in and a/c operation. As you approach 38-42 mph, if you are on level ground, if you are feathering the throttle, if the engine is warmed up, if the car is in third gear, the computer will electronically lock up the torque converter inside the transmission which makes you have direct drive, so to speak.
On this day, the driver brings the car into a shop and complains the engine is bucking and missing. What is happening is the computer sees your throttle pressure stay at 32%, sees the engine rpms stable, but sees the reported speed at 36 then a second later the computer sees 43, then 38, then 41, then 37. The computer would be confused. How can the speed be fluctuating and the engine isn’t and the drivers throttle pressure isn’t changing either? As the speed is reported above 38 mph, the torque converter clutch (TCC) is locked and when the speed is seen below 38 mph, the computer unlocked and disengages the TCC. This rapid locking and unlocking of the TCC feels EXACTLY like an engine miss to the driver. But the computer THINKS the speed sensor is going wacky so it will set the SES light to tell the driver that it sees something wrong.
In this case, the speed sensor was replaced by 3 different shops, except the SES light kept coming on, the same speed sensor code was set and the engine bucked or missed violently when the speed approached 38 to 42 mph. Clearly, the speed sensor wasn’t the problem.
In this case, what finally fixed this car was when we pulled the speedo cable and lubed it. You see when the computer saw the speed vary, it was because the speedo cable was binding up, then releasing causing the speed sensor to report this "speed up, slow down" or 38-41-36-42-34-39 speed reading. Each time the reported speed exceeded 38 the TCC was locked, each time it dropped below 38, the TCC was unlocked causing the bucking or missing. You see, the computer can only guess on the repair, it really can not tell the tech if the speed sensor is bad or the wiring harness connection is loose or bad or if the speedometer cable is jerky and needs to be lubed.
So when your check engine light or SES light comes on, the procedure is the same for all cars. Someone needs to pull the code, check the circuit, test a few sensors, see what’s going on and determine what it is the computer is really seeing and why it doesn’t like what it sees. We typically use a scanner to perform many of these tasks and the scanner will allow us to watch what’s going on. It will allow us to see what the coolant sensor, speed sensor, throttle position sensor is telling the computer. So as we watch the serial stream data, we should know if it is right or not. For instance, if I knew the engine was warmed up but I saw the coolant sensor was telling the computer the engine temperature was -40, I would know the connection at the coolant sensor was defective or the sensor was open. If I saw the coolant sensor was telling the computer the engine was 150F and I knew it was well over 220F, I would replace the sensor, clear the code and test drive the car to see if the code reset or the SES light came back on.
Absolutely everything can and will cause a check engine light or SES light. From a loose gas cap (computer sees a pressure loss in the fuel tank) to a bad sensor or a bad computer (one that has no ability to make things happen or see fault codes the tech creates) , to a vacuum leak, a bad fuel pump, a bad spark plug, a plugged fuel filter or a broken, pinched or disconnected wire.
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