So your check engine light is on. As you probably know, the check engine light (CEL) can light up for all sorts of reasons. But since the introduction of OBD-II back in the 1990s, many of those reasons are stored in a standardized set of diagnostic trouble codes that can be accessed with a generic code reader.
The good thing about the CEL and the entire OBD-II engine management and diagnostic system is that it takes a lot of the guesswork out of what's actually going on with a vehicle's engine and drivetrain. A tech can connect a code reader to the OBD-II port and quickly access any trouble codes that are stored in the engine computer, pointing him or her in the right direction for diagnosis and repair.
NOTE: Code scanning is one of the free services available at your local Advance Auto Parts store.
Where it can get tricky, though, is interpreting what the trouble codes mean. At times, there's a certain amount of reading between the lines needed to come up with an informed conclusion on why a certain code might be stored—and a single problem might cause a cascade of trouble codes to all be stored.
To help you learn more about these codes and decide whether the issue is one you can tackle yourself or one that you should take to a trained technician, Advance Auto Parts will be breaking down common codes. In this article, we focus on trouble code P0125 and what it means.
P0125 Insufficient Coolant Temperature for Closed Loop Fuel Control — What It Means
This is a code that can result from a few different causes and can be difficult to zero in on and fix. Until the engine reaches operating temperature, the fuel system is in "open loop" mode, meaning the PCM alone is responsible for determining fuel delivery. This results in a faster idle, different ignition timing, longer fuel injection pulses and richer fuel mixture (not unlike the long-ago days of carburetors). The engine coolant temperature (ECT) sensor monitors coolant temp, sending information to the PCM.
The engine's coolant should reach about 160-170 degrees F inside of 15 minutes from a cold start. In extremely cold weather (as in sub-zero temps), the temperature of the coolant needs to rise at least 70 degrees from its cold starting temp.
The PCM expects the reading from the ECT to be at a certain value before the fuel system levels off to a normal state and enters "closed loop" mode. In closed loop operation, the PCM's amount of fuel can be fine-tuned by the ECT, the throttle position sensor, and others. A P0125 code means that the ECT sensor is either inoperative, isn't detecting a normal operating temperature, or is sending a reading that's out-of-spec.
Symptoms of P0125 Code
- Poor fuel economy
- Rough running
- Overheating
- Poor performance
- Poor heating or defroster function
- Heightened emissions, smelly exhaust
- Vehicle might possibly not shift into high gear at freeway speed
What Happens If I Ignore It?
Depending on what you're driving, you may not even notice a lot of symptoms other than poor performance and fuel economy. In other instances, the PCM might put the vehicle into limp-in mode, or you could experience overheating that could damage the engine.
Possible Fixes
It's important to remember that the PCM takes in information from the fuel/air monitoring system (namely the O2 sensor) as well as the ECT to know when to put the engine in closed-loop mode. An O2 sensor that has a loose connection, is exposed to an exhaust leak, or has a faulty heater can "trick" the PCM into thinking the engine hasn't reached operating temperature. Toyota in particular has issued technical service bulletins about this problem for certain models. If that's the case, you might see O2 sensor-related trouble codes along with P0125 (more on that in a bit).
Note: When diagnosing this problem, consider a "stone-cold start" as starting the engine after an eight-hour wait.
Check for:
- Low coolant level (for an accurate reading, check coolant level with engine cold)
- Corrosion, damage, loose connection at ECT sensor - you may need to check readings from the ECT sensor with a multimeter to verify whether it's working or faulty
- Thermostat stuck open, never allowing engine to reach operating temperature
- Dirty coolant that contaminates the sensor and causes a false reading
- Defective electric cooling fan that runs continuously
If you also see codes for the O2 sensor:
- Check the sensor for loose connection, damage or corrosion
- Check for an exhaust leak which can introduce oxygen into the system and skew the O2 sensor's reading
- With a multimeter or scan tool with freeze-frame function, monitor the O2 sensor's reading at idle. Normally, the voltage from the sensor should fluctuate a little while the engine is running. If the readings are stuck at low voltage, the O2 sensor could be at fault and additional diagnostic steps should be taken.
In extreme cold, the engine really might just never get a chance to get up to operating temperature. If that's the case, consider a block heater overnight or fasten a sheet of cardboard to the front of the radiator to prevent air flow from keeping coolant temps down. If you determine the problem is due to low coolant level, there's a leak somewhere in the system and you'll need to do a pressure test to try and locate it. You might also find our guide to troubleshooting your cooling system helpful. This is a tricky enough code that you may find yourself needing to take the vehicle to a professional for diagnosis.
Other related codes can include P0115, P0116, P0117, P0118 and P0119, all related to the ECT sensor. O2 sensor-related codes can include P0131, P0132, P0133, P0134 or codes in the P0150 range. As with other problems, it's a good idea to start by clearing the PCM's codes and test-driving the vehicle to see if it enters fault mode again and the code(s) return(s).
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About the Author(s)
Jerry Renshaw is a veteran journalist, gearhead and guitar player, and cultivated his mechanical skills by 30 years of turning wrenches to keep one piece of junk or another on the road. He's owned everything from a Chevette to (three) minivans to a fire-breathing Dodge muscle truck, and is constantly keeping up to speed on what's going on in the automotive world.
