Tip & How-To about Cars & Trucks

My Engine Check Light Is Illuminated

There's nobody on Fixya who can give you a definitive answer as to why your engine check light is on. It just isn't possible, and nobody will tell you to replace parts because some of the parts can be pricey.

If you can understand a number of the causes that lead to a check light coming on, it will go some way in helping you decide what may be the fault:

The Oxygen Sensor
Often referred to as a Lambda sensor.
An oxygen sensor is fitted on the exhaust system before the catalytic converter. It monitors the unburned oxygen from the exhaust and detects how much fuel is being burned at any given time.
A faulty oxygen sensor means it's not providing the right data to the computer and will cause a drop in miles per gallon. Some cars may have more than one oxygen sensor.

Over time, the sensor becomes coated by burnt carbon waste from the exhaust and loses its ability to alter the oxygen / fuel mixture for different driving speeds and conditions. A faulty oxygen sensor also increases exhaust emissions.

A failed oxygen sensor can eventually cause the catalytic converter to burn out. The smell of a burnt catalytic converter is likened to the smell of 'tom cat p**s'. A replacement catalytic converter doesn't come cheap.

Mass Air Flow Sensor
The mass airflow sensor tells the car's computer to add the correct amount of fuel based on the amount of air being drawn into the engine for combustion. A faulty mass air flow sensor also affects mpg, increases emissions, and can cause the car to stall.

Mass airflow sensors can fail over time because of an improperly installed air filter - or an air filter that has never been changed. You should replace the air filter at least once a year to help prevent the airflow sensor from failing.

Spark Plugs
Yes, faulty spark plugs can cause the engine check light to come on - that's because misfiring plugs cause a problem elsewhere in the fuel/air management.
You may feel a 'jolting' when accelerating. Spark plugs fail and there's not much you can do to prevent that from happening.

Fuel Filler Cap
Probably knows as the gas cap in the USA?
Incredibly, an untightened or cracked filler cap can cause problems. Some modern cars even have an error code that suggests a faulty fuel cap.

The Exhaust Gas Recirculation valve.
In internal combustion engines, exhaust gas recirculation is a nitrogen oxide emissions reduction technique used in petrol/gasoline and diesel engines. An EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. It reroutes some of the exhaust gases which are emitted by the engine into the combustion chambers. This effectively reduces temperatures in the combustion chambers.
The exhaust gases from the engine can cause the egr plunger (or 'butterfly valve' - which is a small circular disk inside the throttle body) to stick open or closed, leading to differing symptoms such as engine knock when fuel is ignited too early or rough running. It is the engine timing which is affected.

Most EGRs can be removed and cleaned out. Most EGRs are expensive to replace.

Camshaft and/or Crankshaft Sensor
More often than not when these fail the engine will not start, but this isn't always the case nowadays. Some vehicles have a 'limp home' mode. Replacement of the faulty sensor is the only cure.

There's other sensors too. Your car is a veritable network of data being transmitted from/to a whole host of interconnected sensors.

For the home mechanic the most expensive - and frustrating - way of fixing something is to replace one part after the other in the hope that the problem is solved.

Your first course of action should be to have a diagnostic test carried out. Identify the fault first. It's well worth paying a mobile mechanic to carry out a diagnostic test first.

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1 Answer

rough idle smooths out when the gas is applied

First off is your check engine light on? I know you said your scanner doesn't read OBD1 (pre96) but it wouldn't matter if it did if your engine light isn't on there wont be anything to read.

while it sounds like a misfire it is possible your timing is off.

To be sure, there is a trick you can do to to read the codes if the engine light is on. theres a video on youtube that gives a decent explanation of how to read it, best part is its free. Although Im not sure about the location of the code reading port on your 91, if it isn't in the normal location like the one in the video just google search OBD1 code port location (then year make and model) shouldn't be hard to find.

here's the vid: http://www.youtube.com/watch?v=V8nZpQyoRh4

Sep 01, 2012 | 1991 Chevrolet Lumina

1 Answer

why is my van overheating

First check coolant level, cooling fan and fan relay is working properly, water pump is working fine, thermostat isn't stuck or engine temperature sensor isn't sending right signal.

Hope this helps; also keep in mind that your feedback is important and I'll appreciate your time and consideration if you leave some testimonial comment about this answer.

Thank you for using Fixya, and have a nice day.

Jul 06, 2012 | 1997 Ford Aerostar Extended

1 Answer

Have a 2000 GMC Yukon Denali. Won't fire.

Make sure that the belt isn't extremely tight and they the belt tensioner isn't liked up by pulling in the belt. It should move and spring right back. Make sure engine isn't locked up. And please explain what happen with the vehicle. The fact that you replace plugs and wires and fuel filter for a car that's not turning is throwing me off. So right now we are going all over the place. But definite look at belt and tensioner I've seen a few times that a bad tensioner will make the belt wrap around tighter inhibiting the engine from turning.

Jun 26, 2012 | GMC Yukon Denali Cars & Trucks

2 Answers

Car sputters when I hit the gas then takes off

it's possible the following need to be checked
spark plugs
the air filter
fuel pump

Feb 02, 2012 | 2005 Hyundai Elantra

1 Answer


Very likely the ABS module is bad, this is very common. If you could post the codes that are stored, I could possibly give you a definitive answer.

Jan 25, 2009 | 1999 Volvo S80

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