What doesn't turn the Check Engine light on?

My last diatribe on gasoline octane choice got me thinking. There are situations where engine performance problems do not present any obvious problem to the driver. A knock sensor problem is one of them. There are more.

The fine tuning that engineers do to eek the last few percentage points of power or fuel economy are not discernable to the average driver. The knock sensor is one of those inputs that allow the engineer to maximize power and fuel economy.

When the computerized engine control system diagnostics determine the knock sensor is not functioning correctly it can make do without it. The system will go into a reduced power mode. Ignition timing will no longer be optimized. The knock limit of the fuel is no longer known and thus a less aggressive timing advance curve will be the result.

On many vehicles malfunctioning knock sensors will not turn on any warning lights. The government says the check engine light only has to turn on when emissions will be compromised (one and one half times more than normal). The manufacturer prefers that the check engine light is turned on as little as possible. A reduced power mode will not raise the vehicles emissions. No light!

Will the owner miss that three to ten horsepower? Maybe or maybe not. A good comprehensive tune up will give the owner the chance to get that power back. When your technician queries the computer diagnostic system with a scan tool there will be a knock sensor code.

There are a lot of General Motors trucks, Subarus, and Nissans running around with faulty knock sensing systems. I see them all the time.

Another more noticeable power reducing hidden problem is more common in turbocharged gasoline and diesel engines. It has to do with boost control. Turbocharged engines use a turbine driven by the exhaust system to pressurize the air as it goes into the engine. The higher the pressure, the more oxygen, the more potential horsepower. The term for this pressure is boost.

Boost control systems rely on sensors, solenoids, mechanical wastegates, variable vanes etc.

When the computer controls lose control of the boost the boost is limited to a default level. The default level is usually going to limit horsepower significantly. Many drivers will recognize the power loss but there will be no warning lights on. Limited boost, again, is not going to increase emissions, so no check engine light.

The path to getting the boost back is going to require querying the diagnostic system again. There will be a code and a diagnostic process and most will be happy to get their boost back. It will require some detective work though.

The knock control and boost control systems are two systems that don’t normally turn a light on to let the driver know there is a problem. There are more. A scan of your computer control system diagnostics will likely give the clues required to keep your vehicle performing the way it was designed.