For decades, cars have been equipped with on-board diagnostic systems (OBD). The heart of the system is an engine control unit (ECU). Ending control units have been around for decades. The earlier units were mechanical-hydraulic devices installed in aircraft engines in the late 1930s. They were capable of controlling multiple systems, mainly the ignition and fuel systems and designed to optimize engine performance as well as provide redundancy. Those early aircraft ECUs controlled a system of dual spark plugs for each cylinder. If one spark plug failed, or if the magneto that a controlled that set of spark plugs failed, there was a completely separate magneto and set of spark plugs to provide ignition for the engine. Those ECUs also controlled fuel flow in relationship to the pressures created by the valves in the engine. Modern ECUs, however, are essentially digital computers that monitor a variety of functions by being connected to sensors installed in various places in the engine. They have their origins in attempts to control air pollution by insuring that engines run at optimal efficiency.

In addition to the ECU, modern OBD systems have multiple sensors installed to monitor the fuel injection, ignition, idle speed, valve timing and other main functions. In addition, there are sensors to monitor the position of the camshaft, various temperatures in the engine and exhaust, the amount of oxygen in the exhaust, manifold pressure (MAP), speed, and other functions. The ECU takes information from the sensors and uses it to control a variety of actuators that do things like control fuel flow and fuel pressure, act as a governor for high engine speeds, open and close valves and more. Data from the ECU is outputted through two devices. One is a diagnostic port, where a computer can be connected to read codes produced by the ECU and enable technicians to determine what repairs are necessary for optimal performance. The other is an orange light on the dashboard that usually has a symbol for the engine and the words “Check Engine.” The light doesn’t mean that the driver of the car can see or do anything by opening the hood of the car and looking at the engine. Rather it is an indication that a technician needs to connect a computer to the diagnostic port to determine what repairs are needed. For the most part, there is little that a typical consumer can do to respond to a check engine light other than take the vehicle to a shop where they have a code reader, interpret its data, and make adjustments and repairs. Among the adjustments and repairs is a way of connecting an external computer through the port to set the parameters of the various sensors and actuators for optimal engine tuning.

Operators who see the check engine light do not need to curtail the current trip, but rather should schedule service at the next available opportunity. Most of the time a check engine light appears on the dash not because there is a malfunction of the engine or its major systems, but rather because of a malfunction of one of the sensors. Oxygen sensor are particularly prone to failure. Some of the things that cause the check engine light to appear do not affect the running of the engine and the vehicle can be operated with the light illuminated without causing additional damage.

I had a 1997 Nissan pickup that was prone to oxygen sensor failure. It went to the dealer over and over again and most times what was needed was replacement of the sensor. After many miles and many dollars spent, I learned to ignore the check engine light. I drove that pickup over 40,000 miles with the check engine light shining on the dashboard. When I sold it through a private party transaction, the customer was an auto technician who wasn’t put off by the warning light on the dashboard and paid my asking price willingly.

We also had a 1999 Subaru that I drove over 120,000 miles with the check engine light illuminated. I bought an inexpensive OBD code reader that I could use to not only plug into the port and read the code, which wasn’t consistent, but also to reset the system and turn off the dashboard light temporarily. Since I didn’t really know the meaning of the various codes displayed on my reader, it wasn’t much help, but sometimes I could reset the system and drive without a dashboard light for a couple of days before it returned.

The pickup that we currently own first displayed its check engine light when I was pulling a trailer in northern Montana where there were no dealers nearby. I called ahead on our planned route to the west coast from South Dakota and could not find a dealer who could schedule service to even read the code within a week. Since we were on a vacation with a time schedule, I ended up finding a dealer that could get to the truck at our destination in Burlington, Washington. I pulled five mountain passes with a camping trailer behind with that light shining on the dashboard. The repair was expensive and didn’t last all the way home. We returned from that vacation with the light illuminated. A dealer at our home made a more lasting repair. The light returns periodically, usually at an inconvenient time, and generally is due to a problem in the exhaust system which is pretty complex on modern diesel trucks. The result is that the truck runs and drives well, but is polluting more than optimal. The solution generally is very expensive, costing hundreds of dollars pre-pandemic and generally over a thousand dollars in current post-pandemic inflation pricing.

The light is on again. I have an appointment on Thursday for it to be checked by a shop that I trust. I’ve got my fingers crossed that it won’t be an expensive repair, but won’t know until they connect the computer. I continue to be tempted to just drive the thing with the light on, but I know that isn’t responsible and I want the truck to last for many more miles and many more years.

The cynic in me wonders if the manufacturer has designed the system to fail periodically to keep cash flowing into dealership service departments. I know people who own trucks like mine who have illegally removed the emissions systems to avoid the dashboard lights. I’m not inclined to do that. Still, as much as I understand about the system, it continues to mystify and annoy me. And I’m at the age that I get a bit nostalgic for the cars we owned before ECUs and OBDs and the rest of automotive alphabet soup. In reality they weren’t better than our modern vehicles, but in my memory they seem so.