Anyone who ever forgot to tighten the gas cap on their family car will recognize the look of a Malfunction Indicator Light (MIL). The cryptic warning may not tell a driver the exact nature of a problem, but it certainly identifies trouble with the equipment that is used to keep exhaust levels in check. In the case of a loose gas cap, the pressure has dropped in the system used to control evaporative emissions. But who knows? Something else may be wrong.

Truckers better prepare themselves for lights of their own.

Now that equipment such as Exhaust Gas Recirculation systems is reducing the greenhouse gas that is NOx, and new Diesel Particulate Filters are capturing tiny flakes that would otherwise float out of a truck’s exhaust stacks, regulators are about to introduce new sensors that will tell drivers whether this equipment is actually working.

The MIL and the diagnostic systems behind them will be phased in between 2010 and 2019, and they represent one of the newest challenges to the makers of modern truck engines.

The most popular engines will be affected first. In 2010, the light will need to be included on the dashboards of trucks that are equipped with the best-selling engine in a manufacturer’s most popular engine family. By 2013, the entire engine family will be included in the rules. And by 2016, all engines will need to be equipped with the lights and sensors that come with them, explains Kenworth’s Keith Doorenbos. Warranties for these new warning systems will also need to expand, beginning at five years and 160,000 km, and eventually reaching 10 years and 700,000 km.

At first, the equipment will need to spot failed components that cause a five-fold increase in emissions, but the tolerance levels will tighten with each passing year.

“These rules are actually quite complex for those of us who are manufacturing and have to go through the certification process,” says Kevin Otto of Cummins.

Consider the different sensors that will be needed to power such a light. Sensors will need to watch over exhaust-cleaning components such as the EGR valves and any NOx-reducing catalysts. (The latter equipment could be required to slash NOx to levels that are even lower than those that exist today). Engine makers, meanwhile, will need to use these sensors to generate fault codes about specific problems, says Doorenbos. Some of the monitors will run all the time, while others will need to run once per “drive cycle” – a period that includes 10 minutes of running time, 30 seconds of idling, and five minutes of operating time at a 15% load.

The sensors themselves will need to work in a few different ways, adds Tim Gundrum, who is responsible for on-board diagnostic certification and compliance at International Truck and Engine. An “out of range” sensor, for example will help to identify readings that don’t make sense – such as an ambient temperature sensor that says it’s 200 F outside. Rationality monitoring (also known as “in-range diagnostics”) will compare a sensor’s reading to other information that should come with it, such as the increase in boost pressure that should accompany higher engine torque. If the voltage on a related sensor does not jump high enough, the diagnostic system will understand that there is a problem with the sensor.

All of this data will be delivered over a proven communications network, such as the J1939 systems used on today’s truck engines, or the OBD-II systems on your family’s car.

“(Exhaust Gas Recirculation) diagnostics are not new. Many of these systems have had these in place,” says Greg Gillham, manager of on-board diagnostics with Detroit Diesel. Indeed, the sensors on a 2007 engine will monitor whether EGR rates are too low or too high. But there is a difference. In 2010, the sensors will need to identify specific issues such as a stuck EGR valve that will allow an engine to spew an excessive amount of particulate matter, or the high intake temperatures on an EGR cooler that can lead to higher NOx levels.

The oxygen sensors found in today’s passenger vehicles will also have a heavy-duty diesel equivalent in a future NOx sensor – something that will be particularly important as NOx levels drop from 2.5 to 2.0 grams per brake horsepower hour.

This is where things get really tricky. The more sensitive the diagnostics become, the greater the chance for a false reading, explains Cummins’ Ben Zwissler. A simple drop in pressure can identify a cracked or melting Diesel Particulate Filter, but this condition could also be caused by the simple accumulation of soot or a change in temperature.

“The OBD rules will really add a significant amount of regulation to the industry. That is going to affect vehicle maintenance,” Otto warns. “Understanding how these very, very complex diagnostic systems work will be critical if you’re going to do a good job in troubleshooting.”

The good news for fleets is that the pending rules require service tools that can read fault codes on any model of engine. Still, clearing the fault codes may be easier said than done. The lights will remain lit for three driving cycles unless they’re cleared by a service tool, says Jim Roal, Caterpillar’s diagnostic technical steward. Some monitors will also require road tests to determine whether a troubled component has actually been fixed.

Technicians will also need to rely more on the sensors to identify related faults, adds Zwissler. “(You) will have to trust the engine to tell you what’s going on.”

Drivers, meanwhile, will have to trust the technicians. The performance of the truck may seem unchanged.

The only sign of trouble will come in the form of a glowing dashboard light.