Q: What are the common things to check when the check engine light comes on?

asked by on

what are the common things to check when the check engine light comes on?

My car has 100000 miles.
My car has a manual transmission.

When the Check Engine Light illuminates, it indicates that Diagnostic Trouble Codes(s) have been stored in your car’s engine computer. The codes themselves have to be downloaded in order to know where to begin the diagnostic. The most important thing for consumers to understand in interacting with their mechanic is that the codes do not necessarily identify the failed part, or indeed any failed part.

Codes are best thought of as the consequence of the engine computer not getting the input (data or voltage) that the computer "expected" or perhaps no input at all. This is important because although it is often "sensors" that are supplying the data to the engine computer, just because the signal has a problem, it doesn’t mean that the sensor (referred to in any given "code") is bad. Instead, it could simply mean a wire is broken, or there is a bad connection to the sensor or it could even mean that some other malfunction, that is NOT even being reported as a "code" is causing the sensor to start reporting "out of range".

So, to answer your question, first the codes have to be downloaded but those codes just provide "clues", albeit important and critical, from which the mechanic starts his or her actual diagnosis. In practical terms, this means that any "device" identified in a code has to be separately tested before its status (whether it works or not) can be determined.

Regarding your question of common things to check, there are many studies of the frequency of malfunction codes and they all have different results because there is some variance from car model to car model. However, one major study showed that the top five reasons for the Check Engine Light to illuminate were traceable to these codes: P0420 - Catalyst System Low Efficiency, P0171 - Fuel Trim System Lean Bank 1, P0401 - Exhaust Gas Recirculation (EGR) Flow Insufficient, P0174 - Fuel Trim System Lean Bank 2, and P0442 - Evaporative Emission (EVAP) System Small Leak Detected.

These five codes are not uncommon faults but again, your experience at the moment could be completely different. If your Check Engine Light has illuminated, it is recommended that you schedule a diagnostic with a certified technician, such as one from YourMechanic.

Was this answer helpful?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. What are the common things to check when the check engine light comes on?

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: My check engine light came on and the it was diagnosed as op128 so I got gas screwed on my cap and my light turned off how is this

First, a trouble code P0128 does have to do with coolant, but it is not the level. This code is set when the relationship between the coolant temperature sensor and the intake air temperature sensor don't increase at a predetermined...

Q: Q: The transmission is slipping.

Hi, thanks for writing in. It seems that the clutch disc is most likely worn to the point that the pressure plate cannot fully engage and lock the clutch to the engine. If that is the case, you will need...

Q: Coolant flow control valve and camshaft position

A camshaft position sensor is a sensor that monitors camshaft speed to regulate ignition timing and fuel injection timing. This is a small sensor that is usually easily replaced and fairly inexpensive. The coolant control valve controls the flow of...

Related articles

How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
P2428 OBD-II Trouble Code: Exhaust Gas Temperature Too High Bank 1
P2428 code definition A P2428 trouble code signifies that the PCM has detected a problem in the exhaust gas temperature sensor circuit in bank 1, which subsequently contains the number one...
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.