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The check engine light always gives that sinking feeling in the pit of your stomach. For many, it means a long-avoided visit to the mechanic. Having this warning light illuminate on your dash, followed by an error code, is already stressful. But what if you see an error code but no light?
The problem that triggered the code may not have happened enough times to illuminate the light. This most likely means the issue is not critical and that the code is pending. So if the check engine light is off, but you see an error code, use an OBD2 scanner to help identify what the problem is.
An OBD2 scanner may also provide playback features and testing equipment. If you’re unable to identify the code and fix it, visit a mechanic.
Read on to learn more about this problem and its resolution.
The Dreaded Check Engine Light
Originally known as the idiot light or Malfunction Indicator Lamp (MIL), the Check Engine Light (CEL) is used by the Powertrain Control Module (PCM) to alert you of a problem with your vehicle. For most cars, the CEL is found on the instrument panel or dash and displays the following:
- Check engine
- Service engine soon
- Maintenance required
- Emiss Maint
- It may also show a pictogram of an engine
It usually gives off an orange, yellow, red, or amber color and could either be flashing or steady. This warning light can mean anything from low oil pressure to an imminent engine breakdown.
Along with turning on the CEL, the PCM also stores one or more corresponding trouble or fault codes in its memory. Before the introduction of OBDII, the check engine light on most cars could output codes. Now, you will need a scanner to identify the specific code.
Typically, the check engine light illuminates after the PCM has identified a problem and stored a trouble code. But in some cases, you can get trouble codes, but the light does not come on. Or the CEL has come on, the fault has been addressed, and the light is now turned off, but the scanner is still reading codes. Either situation can cause unwanted anxiety, so it is crucial to identify the root cause of the occurrence and stop it.
What Triggers the Check Engine Light?
Knowing what activates the CEL is a must to understand why it is not coming on. Your vehicle’s PCM/ECM/ECU has stored tons of trouble codes in its repository, pointing to different triggers. Here are the most common ones:
- Loose gas cap – A loose cap disrupts your car’s fuel delivery system as it fails to keep the whole system under the correct pressure and results in fuel loss. Tighten the gas cap or replace it, then drive your car. The light should then turn off by itself, although there is no specific time when this will happen.
- Oxygen sensor failure – A bad O2 sensor can lead to reduced fuel economy, damaged spark plugs, and a failed emissions test. It is also indicative of high amounts of methanol/ethanol or other additives in your engine.
- Spark plug and ignition coil issues – Worn or fouled spark plug, wires, and ignition coils can cause an engine misfire, impeded acceleration, lower gas mileage, and an unexpected shut-off.
- Defective catalytic converter – Poor vehicle maintenance like oil changes or driving only short distances most of the time results in a clogged catalytic converter.
- Mass airflow sensor failure – This sensor monitors the amount of air entering the engine and enables your vehicle to adjust to altitude changes. When defective, its symptoms include rough idling, reduced gas mileage, and sudden changes in the throttle pedal position.
- Vacuum leak – The vacuum system performs a wide variety of functions, such as lowering harmful emissions. Your vehicle will start to surge or settle at an unusually high RPM if there’s a leak. It usually traces back to cracked fittings, loose connections, or the vacuum hose drying out.
- Thermostat issues – The thermostat regulates the flow of coolant to the engine. It contributes to a contaminated coolant when defective or with a leak, which could turn corrosive, leading to an overheating engine. Adding coolant does not reset your CEL, nor does it guarantee the thermostat or ECT sensor is damage-free.
- Low Oil Pressure – This problem requires immediate repair as overfilling or air in the oil pump can disable your vehicle. Your oil light is lit along with the CEL to indicate this issue.
These issues typically trigger the check engine light. However, this does not mean you can rule them out if your scanner receives codes (without the warning light coming on). If you experience any of these issues but still get codes after repairs and the light turning off, you either have a CEL glitch or haven’t fully addressed the problem’s cause. Consult a mechanic in this situation.
One concern of vehicle owners is that the CEL illuminates intermittently. At times, the check engine light is off, but the code is still there. On other occasions, it works fine and lights up along with an error code. A possible explanation for this is that the PCM may have a poor electrical connection or wiring problems causing the intermittent CEL function. Other potential causes include engine sensor failures or stuck valves.
Whatever the cause, it is best to pop the hood and visually inspect all wires, cables, fuses, and other types of connections. You can also try to narrow down the problem. For instance, if you notice CEL issues only during a downpour, then replicate a rainy day using a hose. If all else fails, have a mechanic perform a CEL diagnostic to identify the engine controls that need fixing.
Engine Code Classifications
In addition to CEL triggers, you also need to get acquainted with the kinds of codes your vehicle throws off – the standard and enhanced codes – and how to interpret them correctly.
Standard or Generic Codes
This set of standardized diagnostic codes, a.k.a. “P” codes, covers engine-related vehicle systems, is readable on any OBDII-compatible vehicle, and has three types, namely:
- Pending Codes – These codes will not activate your check engine light but will warn you of a system on your vehicle behaving abnormally. For instance, a temperature sensor with a standard operating range of 90-110° that reads 85° may give out a pending code without triggering any warning light, simply because temperature sensors usually have an acceptable range of 80-120 degrees.
- Confirmed Codes – Taking the same example above, once the temperature sensor reads anything below 80 degrees or above 120 degrees, your vehicle would then throw this code out. Unlike pending codes, this is accompanied by an illuminated warning light and would mean an issue requiring your attention.
- Permanent Codes – Introduced more recently, this particular code will only clear itself once the cause of the issue has been addressed and cannot be cleared with a scan tool. This is because the PCM will only clear the code once enough data is gathered under different driving conditions. Permanent codes generally trace back to emissions-related equipment.
Enhanced or Manufacturer Codes
These codes cover ABS, airbags, transmission, BCM, HVAC, transfer case, and other non-engine related vehicle systems. They are unique to every manufacturer and are inaccessible on generic scanners. Enhanced codes are split into four types:
- Powertrain (P)
- Body (B)
- Chassis (C)
- Network (U)
Both code classifications have an alphanumeric format. A letter represents the first bit, while the rest are all numbers. Interpreting the third digit is vital as it points to the vehicle part with the issue. It is represented by numbers 1-8, which stand for:
- 1 – Fuel and Air Metering and Emissions Control
- 2 – Fuel Metering
- 3 – Ignition System and Misfires
- 4 – Exhaust Monitoring
- 5 – Vehicle Speed Controls and Idle Control System
- 6 – Computer Input/Output Circuit
- 7 & 8 – Transmission
Deciphering Check Engine Codes
Now that you’re familiar with the different error code types, let us move on to decoding them. The oxygen sensor code P0171 is a good place to start.
Here’s the breakdown of this example:
- P – The first character indicates the vehicle system affected. In this example, P stands for Powertrain.
- 0 – The second one denotes the error code type. 0 is for standard/generic while 1 is for enhanced/manufacturer.
- 1 – The third character highlights which part of the vehicle is problematic. In this case, 1 represents Fuel and Air Metering and Emissions Control issue.
- 71 – The fourth and fifth digits indicate the fault code, which means a lean air/fuel mixture for this example.
If the second digit shows a 0, there’s a 50% chance that your check engine light will not activate as it could be a pending code. If so, make sure to address the issue indicated by the engine code. Remember, codes can still be read even with the CEL turned off. The absence of the warning light does not make the problem negligible.
How to Reset Your Check Engine Light
While newer vehicles have a self-correcting mechanism that switches off the CEL once you fix the root problem, there are situations when you need to do it yourself. These include the car not doing so on its own, avoiding an emission test failure, or being 100% sure there is no problem with your vehicle. Here are the steps on how to reset the check engine light:
- Drive until it turns off. Your vehicle’s sensors generally recheck the problem that triggered the light and turns it off once it recognizes that you have fixed the issue. This process is put into motion when you drive your car.
- Turn the ignition off and on three times. Some call this the PCM/ECM hard reset technique. The vehicle resets the trouble codes by consecutively turning the ignition key on and off. Since this is quite simple, it is best to try this first before moving to other ways of resetting the check engine light.
- Disconnect the Battery Terminals. Pop the hood, disconnect the battery’s negative cable, press the horn to drain any remaining electricity from the capacitor. Then wait 15 minutes. Put your key in the ignition and turn it on and off three times to clear emission and flag data. Then reconnect the negative cable and start the car. The light should clear in 1-2 minutes. Do this at your own risk, as reconnecting the battery may reset all your radio and onboard component settings, along with the light.
- Use an OBDII Reader. You will need a code scanner to reset your vehicle’s OBD computer – although not all code readers can clear the codes. If you happen to have one capable, then it’s as easy as plugging it in and selecting which code to remove. Note that automobiles produced before 1996 may require you to use a different scanner or may not have OBD ports beneath the steering column or control panel at all.
- Determine which fuse controls the light. To remove the check engine light, you can pull off the PCM fuse and replace it or disconnect the battery to disable the CEL fuse. This method may not be useful for newer cars, as it will simply disconnect the CEL connection with the electronically controlled unit.
Does the Check Engine Light Reset Itself?
Once you repair the problem in most car models, the check engine light will reset. However, it’s not instantaneous. Each component is given its priority in engine management. Depending on the problem, the check engine light goes off after passing a test for at least three times. The light will reset itself after 50-100 miles of driving or 10-20 successful cycles for more severe issues. A cycle relates to starting your car cold, driving it until it is warm, and ending only when you finish driving. Completing this number of cycles does take time and patience. But if you’re not willing to wait, reset the check engine light with an OBDII scanner like Autel MS906 MaxiSys OBDII Diagnostic Scanner (view on Amazon).
Frequently Asked Questions
- How much does it cost to diagnose a check engine light? The average fee to have a mechanic diagnose your check engine light is $100, but you may find cheaper, not include repair costs. The good thing is that you can preview the potential problem yourself by purchasing an OBD2 code scanner (view on Amazon) online or from an auto parts store. Most modern systems will display the code through an app on your mobile or smartphone.
- How long will the check engine light stay off after reset? This depends on whether you permanently fixed the root problem that activated the check engine light or if other vehicle failures would trigger another warning code.
- Does resetting your check engine light pass emissions? If you do the reset last-minute, you are guaranteed to fail your emissions test. The reason for this is that your vehicle’s catalyst and EVAP monitors need time to run according to spec. The best way to ensure passing emissions is to pre-test your vehicle ahead of your test schedule.
Conclusion – Check Engine Light Off But Code Still There
Resolving trouble codes and figuring out why your check engine light may not be working can be frustrating, especially if you are not savvy with how your PCM works. Luckily for vehicle owners, all automobiles manufactured in the past two decades are OBD-compliant. Nowadays, having an OBDII scanner and a smartphone helps identify the triggers for codes and resolve engine failures.
In cases where there’s no check engine light but codes, don’t forget that non-emissions related problems typically do not set the check engine light on. There are also types of errors like pending codes that do not require this warning light. Remembering this fact will help you stay clear-headed should you encounter this problem on the road. Additionally, familiarizing yourself with your vehicle’s scheduled maintenance needs will help prevent issues that cause the check engine light to come on in the first place.