My Toyota RunX has a ‘check engine’ light on throughout

Dear sir,
I own a Toyota Corolla RunX, 2001 model. Recently, the ‘Check Engine’ light came on as the engine was idling. Since then it has remained on whenever the engine is running.

Sometimes it goes off, but for a very short time. At first I thought it was the faulty battery I had which had problems cranking the engine, but after I replaced it, the problem persisted.

The car runs well, there are no funny noises from the engine, even at highway speeds. I haven’t taken the car to a mechanic (I’m low on cash right now), but I’m still bothered. Is there any cause for alarm?

Kenneth.

The best way to know what that ‘Check Engine’ light is all about is to do a diagnosis. However, these here are common causes of the light coming on out of the blue:

1. Faulty Oxygen Sensor: the device is not transmitting accurate information to the ECU and this is sometimes accompanied by a reduction in fuel economy. Cars have two to four of these sensors: the OBD code will identify the culprit for you. The cause is this: over time, the sensor gets covered in oil/soot and thus does not determine the quality of oxygen in the exhaust properly.

2. My research leads me to a very strange cause: a loose or cracked fuel filler cap. Apparently leakage of fuel vapour from the tank can very easily confuse the entire fuel system. This is also accompanied by worse fuel economy. Check the filler cap for cracks, or remove it and tighten it again, then drive a bit to see if the light will go off.

3. Faulty Catalytic Convertor: Failure of this device can be caused by 1 above (a faulty oxygen sensor makes the car run rich and this fouls up the plugs and the cat. Fouled plugs can be caused by a faulty oxygen sensor too. As you can see, these problems can sometimes be interconnected in a veritable web of complexity)

4. Faulty MAF Sensor: This is NOT the oxygen sensor as some are wont to believe. The oxygen sensor senses the amount of unreacted oxygen in the exhaust and adjusts the timing accordingly to optimise economy and reduce emissions. The mass air flow sensor senses the amount of air going INTO the engine and instructs the ECU to meter out the fuel accordingly via the injectors.

MAF sensors tend to fail because of a badly installed or rarely-replaced air cleaner element. A once-annual replacement of the air cleaner is just about enough to keep the sensor from failing.

5. Weak Electrical Connections: Plugs and wires in particular. This is usually accompanied by the vehicle jerking while in motion. Since you have not mentioned this, we can leave that at that. Only Part 3 would cause you to worry because cats are expensive to replace and require specialised skill to install.

Hi, I hope you enjoyed your trip down south. I confess I did not take your advice to sell my Mitsubishi Chariot when it started giving me trouble. I had it repaired and, despite the cost of having to change several sensors, I still kept the car.

Call me names, but I had become accustomed to its comfort. Now, the mother of all problems has come up; the gear is stuck at Three. I have had several diagnosis from different mechanics until my head is now spinning, but none of them has been able to solve the problem.

I have sworn the moment the problem is solved I SHALL SELL it. What do you think could be the problem? This time I promise to heed your advice.

Margaret (@MachariaWanjiru)

To reduce guesswork, obtain a code from the TCM (Transmission Control Module). This will give you a code from which you will know exactly what the problem is.

Usually this 3rd-Gear drive is the fail-safe, limp-home mode, which is usually triggered whenever the TCM receives an electronically generated error code. In case you cannot communicate with the TCM, then therein lies your problem: the TCM itself is cooked.

The transmission may have to be opened. A coil pack may have failed and overheated from an electrical surge, melting the module. Mitsubishi, for some reason, thought it wise to place the two in close proximity to each other. If this is the case you are facing some major (and expensive) repairs. I can bet the mechs will tell you: “Nunua gearbox ingine, Mummy!”