Do codes come up for cars with no CEL (engine, replace, sensor)

[TAZORAC]

Is it possible for codes to come up on a car with no CEL on?

[eddie1278]

Yes on some makes such as GM and Ford there is what's called mode 6 data an advanced diagnostics. It will tell you what sensors or other components are ready to set off a check engine light before it does.

[d4g4m]

The mode 6 thing is for when you bring the car in for the $29 oil change and they tell you that a $39 sensor will be going bad and there's a $329 charge to replace it. [that takes 15 minutes]

[NY_refugee87]

Yes and it's usually not a sensor.

Have seen far too many UTI wonder boys change a NOX 1/1 sensor and NOX Module for a faulty EGR or EGR bypass in a 6.7

[Katana49]

Short answer, yes. A code that starts with a P that is universal can turn your CEL on. Other codes can also turn it on obviously, but any "P" code will do it.

However, a manufacturer specific code won't necessarily trigger a CEL. My cars can throw all kinds of codes that will show up as a warning message on the dash, but will then go away until the next time the car is started. Really just depends on the car.

[DeanJ44]

OP: Yes. The computer can store a "pending" code that either has not met the conditions enough times to light up the CEL, or if the condition had previously lit up the CEL, but has since gone away - if there haven't been enough cycles to completely clear it from the computer. That is exactly the situation I am dealing with right now. I have been having an intermittent misfire causing a P0300 code but the CEL was only on for a couple of days and now it has been out for about a week. The misfire condition continues. I had the codes read again yesterday to see what was there hoping for a more useful and specific clue - and the p0300 was still pending in the system.

[BostonMike7]

Also, Pre-OBD2 cars with early check engine light systems don't always toss a CEL when a code is present.


I'm familiar with Ford's EEC-4 setup (OBD1) and only 13 or so of 100 potential codes will cause the light to come on.

[TAZORAC]

Sweet.

[Ollie4]

Yes.

They are called "Pending."

Meaning that it only a matter of time before they cone back on.

Low Catalytic Converter Efficiency code is great example, it pends before it sets....

[papilgee4evaeva]

One example: on 4th-gen (95-99) Nissan Maximas, the code P0325 (knock sensor malfunction) can show up without turning on the CEL. They call it a "ghost code" because unless you have other codes that do trigger the CEL, you don't know that it's there.