UNDER THE HOOD Getting to the bottom of ‘check engine’ light

Q:I read your column on a regular basis in the Philadelphia Inquirer. I have a dilemma. I own a Toyota 2007 RAV4 with 64,785 miles on it and maintain the maintenance on it. I periodically get a computer indication (picture attached). I have taken the vehicle in when this comes on and a reading was taken. I was told to add injector cleaner to my gas and to also clean the electric sensor above the air filter compartment with an electronic cleaner. I did this and it still stayed on. After awhile, it went off. It stayed that way for quite awhile and then came on again. I didn’t do anything that day and the next day it was gone. I’m scratching my head. Your input is greatly appreciated. —Alan J.M.

Alan, your photo shows the “check engine” light is on. This indicates your RAV-4's on-board diagnostic system (OBD-II) has identified a condition that infers emissions will rise above an approved level. There are perhaps a hundred or more possible causes, as many parts and functions are scrutinized either on a continuous or occasional basis as the RAV-4 is driven. Should a fault arise a diagnostic trouble code (DTC) will also be set, and remain in memory for either 40 or 80 warm-up cycles, depending on severity.

Some faults will cause the light and code to set the first time the abnormal condition occurs; others require two consecutive incidents to occur. Intermittent faults such as yours can be frustrating to fix, as the fault needs to be present at the time of testing for an accurate diagnosis to be performed. If the fault goes away, the OBD-II system will turn off the light after three consecutive good trips (a drive during which the appropriate test is run) and hold the code in memory.

I'm disappointed the folks you took the RAV-4 to didn't indicate the code and provide a little more information as to possible causes. You can purchase an inexpensive OBD-II code reader and pull them up yourself ($20 or less on Amazon). You can also pick up an ELM 327 device for under $10 that can be paired with a free or $5 smart phone app; these may also provide limited system data. Once the code is obtained, you can go to obdii.com to look up information on the possible causes.

Resist the temptation to throw parts at the problem based on codes! Let's take your mass airflow sensor for example (the sensor is a black thumb-sized cartridge that plugs into the air intake duct). There could be a MAF code indicating a range/ performance fault that could be caused by an external problem such as an air leak or conflicting information from other sensors, a dirty sensing element or a fault within the sensor itself. Other MAF codes might indicate a circuit fault (high, low, intermittent signal), possibly caused by a loose or corroded circuit connection, or rodent damage to the wiring. A fuel trim code (too lean or too rich) could also set because the readings from the exhaust oxygen sensor(s) are in conflict with the MAF sensor's perceived airflow, or another fault is present.

In a few cases, such as a P0135 oxygen sensor heater code, with a high probability the fault is within the sensor, and the sensor may be inexpensive, it may make sense to throw this part at the problem rather than pay for perhaps an hour's diagnostic time (typical minimum) to have it diagnosed and repaired. Or in the case of a P0440 EVAP system leak, tightening the gas cap and driving the car for a week or more might extinguish the light.