Check Engine Light After Oil Change – 2 Min fix


Check engine light on can be alarming, there’s literally never a good time for a breakdown. But don’t panic, I’m a mechanic and you are in the correct place, in a few moments you’ll be well on your way to fixing this minor issue.

A check engine light after an oil change is commonly caused by fitting the oil filler cap backways or oil dipstick not fully seated. Removing and fitting the cap correctly, or seating the dipstick will fix the issue. The engine light may not go out immediately, but likely will within three engine start cycles.

In this post we’ll cover the top causes of the engine light and how to fix them in a little more detail. In addition you’ll learn other possible causes of an engine light after an oil change, how to diagnose and fix them.

Oil Cap Fitted Backways

Sounds almost too simple, right? How could fitting an oil cap backways cause a check engine light?

The reason becomes clear once you understand what’s going on. Stay with me here, this will be the light version, I’m guessing you want fix the problem and be on your way.

That said, the short answer is:

Your cars engine requires a precise ratio of air (oxygen) to gas in order to run at it’s optimum. It’s known as the Air Fuel Ratio (AFR). It’s 14.7 parts air to 1 part gas.

Your cars on board computer (Engine Control Module or ECM) is set the task of managing this ratio. It does so by measuring the volume of air entering the engine through a sensor located in the air intake system. It’s known as a Mass Airflow Sensor or MAF.

As the ECM knows how much air has entered the engine, it can make a simple calculation of how much gas it needs to add, easy work for a computer.

Problems arise however, if extra oxygen sneaks into the engine without being accounted for by the ECU. And that’s exactly what happens when some oil caps are fitted backways. The cap doesn’t seal completely and creates what’s known as a vacuum leak, which causes the engine to run lean.

Lean simply meaning the AFR has too much air in the mix.

A lean condition won’t fool an ECU, it’s always on the look out for problems. It uses a probe in the exhaust to measure how well the fuel it’s adding, is being used. It’s checking its own calculations if you like. The exhaust sensor it employs is known as an oxygen sensor.

Chevrolet engine

The ECU has a range of readings it will accept from the oxygen sensor, given the ratio of fuel its added. If the oxygen sensor readings are outside this window the ECU sets a warning by illuminating the check engine light.

The fix – remove the cap and fit it correctly. Correctly by the way, likely means the cap writing faces you, I know that sounds stupid, but for some vehicles it’s the solution.

You might well ask, why would the manufacturers design an oil cap that can be fitted backways, oversight, and having worked for many brands oversight is soooo common at even the premium brands.

Dipstick Not Fully Seated

oil dip stick

The dipstick is as you know used to check engine oil level. At an oil change, the dipstick will be removed, cleaned and reseated several times to check the oil level.

The dipstick fits into the dipstick tube and an O-ring creates a seal when the dipstick is pushed home. Leaving the dipstick off it’s seat will allow unmetered air sneak into the engine, and you know the outcome of that.

I’m not making excuses here, merely offering an insight. Many mechanics are paid by the job and performing oil changes really doesn’t pay very much. Indeed most shops don’t make a ton on oil changes either, they are seen as loss leader. This often results in rushed oil change jobs, they are pushed through the workshop quickly, so the shop can get onto the higher paying brake jobs etc.

The fix – pop the hood, locate the dipstick and seat it fully.

Oil Level Overfull

Too much oil inside the engine may set a check engine light, it may even cause the engine to feel sluggish, misfiring, smoke from the tail pipe, oil leaks or worse not start at all. Yep, too much oil is bad, it’s not as bad as too little of course. As oil problems go, you definitely want the too much oil one.

How to check engine oil level

Oil level on dip stick

Check the oil level as follows:

  • Park on level ground
  • Allow engine rest a few minutes before checking
  • Pop hood
  • Locate dipstick
  • Remove and clean dipstick with clean cloth
  • Refit and remove to read

Your dipstick will have an upper mark, likely marked “Full”, “F”, “Max” or a notch, it marks the optimum oil level.

Towards the bottom of the stick, it will likely be marked “Low”, “L”, “Min” or a simple notch, this marks the oil critically low level.

An area between the the upper and lower marks on your dipstick may be hatched. An oil level in the hatched area is at an acceptable level, it’s neither critical nor optimal.

How to remove excess oil?

Oil siphon

If you find you have too much oil, you’ll need to remove the excess. Obviously, taking it back to the garage to the garage that performed the lube is the way to go. Failing that, I’d syphon oil from the engine through the dipstick tube. You will require an oil syphon, there not expensive and are a mess free way to solve the problem.

You can check out the syphon I use here on the “Mechanics tools page”.

Other Possible Causes Of Engine Light

We are at a point where we are simply guessing what the problem might be. A code reader would be an invaluable tool in this instance, it would remove a ton of guess work.

Code readers aren’t expensive, they’re simple to use and as many problems can be fixed by even the novices, a code reader can save you a ton of money. If need a code reader check out my review of the Topdon code reader.

Without the code reader, we’ll push on with the guesswork. I’m basing this list on things that could cause a check engine light after an oil change, rather than are likely to have happened. Make sense?

Here’s a list:

  • Oil cap left off
  • Oil spill on sensors
  • Sensor wiring disconnected
  • Evap wiring loose
  • Loose gas cap
  • Faulty gas cap

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of classic car ownership, from tires to roof aerials and everything in between.

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