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Check engine light on no codes

Check engine light on, but no codes stored

Check engine light on but no codes show up

Many people post these questions in auto forums:

“Why is my check engine light on?”
“My check engine light is on. What’s wrong?”
“My check engine light is on but no codes show up”

They’re all looking for an answer as to why their check engine light is on but no codes appear. Here’s the answer.

Code readers can’t read all trouble codes

The On Board Diagnostic II system was set up to assign a standard list of trouble codes for all auto makers to use. Prior to that, car makers could make up their own code numbers and definitions. After the introduction of OBDII in 1996 it became clear that the standardized codes weren’t enough to really nail down the source of the problem. So auto makers began introducing their own set of trouble codes than became known as manufacturer-specific “enhanced” codes.

Generic trouble codes

Generic OBDII powertrain trouble codes start with the letter P and the four digit number 000o and go to P0999. Low cost code readers can read all these codes. But the low cost models usually can’t read enhanced codes.

What are enhanced OBDII codes

Car Makers are allowed to use enhanced codes that start with P1000 and go to P2999. Do NOT confuse these P1xxx or P2xxx codes with generic codes. Plus, even if your scan tool can read enhanced codes, don’t be tricked into thinking you can just do a search for that code on the internet. Enhanced codes are vehicle-specific and may vary from one model year to the next. So a P1252 trouble code may mean “Pedal Correlation PDS1 and LPDS High” on one car maker’s model, but something else on the same car maker’s other model.

at’s more, some types of faults will set a P1 code but not set a P0 code. That’s why you need a tool that can read both types of codes, not just the generic ones.

What scan tools can read enhanced trouble codes?

Generic trouble code definitions are free, but car makers are allowed to charge for enhanced trouble code definitions and those definitions constantly change from year to year and model to model. If you buy a scan tool or code reader that claims to read enhanced codes but the software can’t be upgraded, you could easily find yourself in the position of having a check engine light without seeing any trouble codes.

Then there are B, C and U trouble codes

B Body codes
C Chassis codes
U Communications codes.

Car makers use “B” body codes for airbag, power seat, HVAC and other body systems. “C” codes report problems with brakes and suspension or “chassis” related faults

U codes relate to problems with body computers and digital communications between computers.

So, your check engine light on with no codes doesn’t mean there are no codes stored

If the check engine or service engine soon light is on, trouble codes are stored. PERIOD. If you attach an inexpensive scan tool or have the auto parts store scan your car and you come up with no codes, you should always assume that the scan tool wasn’t capable of reading B, C, U and enhanced codes.

Why is the check engine light on?

Engine computers monitor many sensors and the computers are programmed with a table of acceptable readings from each sensor. When a sensor reports a reading that’s outside the table, the computer will flag a particular trouble code, store it in memory, and turn on the check engine light (or service engine soon light). See this post for more information on trouble codes.

To find out why the check engine light is on, you MUST use a scan tool or code reader on OBDII cars (made after 1995). On OBDI cars (made before 1996) some can be read with a code reader or by triggering a bulb flash sequence. That usually involves jumpering between two test terminals and then counting how many times the check engine light flashes.

Powertrain codes

The folklore says that the check engine light only pertains to emissions related problems. That’s simply not true. Here are some examples of powertrain codes that are NOT strictly emissions related

P0223 Throttle/Petal Position Sensor/Switch B Circuit High Input
P0237 Turbocharger Boost Sensor A Circuit Low
P0320 Ignition/Distributor Engine Speed Input Circuit Malfunction
P0500 Vehicle Speed Sensor Malfunction
P0560 System Voltage Malfunction
P0569 Cruise Control Coast Signal Malfunction
P0620 Generator Control Circuit Malfunction
P0707 Transmission Range Sensor Circuit Low Input
P0731 Gear I Incorrect ratio

But computers can set other codes besides Powertrain (P) codes. They can also set

B Body codes
C Chassis codes
U Communications codes.

If you attach a cheap scan tool, like the ones used at auto parts stores, and the tool is only capable of reading P codes, you’ll find yourself in a situation where you have a check engine light, no does. You may think it’s a computer glitch. Possible. Not likely. Chances are far more likely the scan tool or code reader isn’t capable of reading B, C, or U codes.

At that point you’ll need to get the computer scanned by a real professional scan tool

DON’T post on a forum and ask us to guess why your light is on if you haven’t done your homework by getting the trouble codes read. It’s a complete waste or our time. There are simply thousands of possibilities. And the bottom line is this: If you’re too lazy to get the trouble codes read, why should we waste our time trying to help you.

The other point is this: If you plan to do your own car repair, you need the proper tools. Besides sockets, ratchets, and wrenches, a scan tool is a must have item if you’re going to fix your own car. In the old days you invested in a timing light and tach/dwell meter. So stop whining about how new cars need new tools. Either you want to invest in the tools so you can save repair costs or you don’t. But whining isn’t acceptable.

©, 2015 Rick Muscoplat

Posted on by Rick Muscoplat

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