Will Limp Mode get you Home

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What is Limp Mode?

My wife and daughter made a long drive from Seattle, Washington to Ashland, Oregon about 20 years ago and just as they were coming into Ashland, the "Limp Mode" icon appeared menacingly. She called me and asked what it meant. I explained the concept of limp home mode and asked if she noticed anything unusual such as the engine running rough, the revs being limited, or if the transmission was restricted in some way. Turns out the transmission was indeed restricted, and I suggested she take the car to a mechanic right away.

It seems the transmission fluid was low and had overheated. Luckily, we caught the problem in time before the tranny was burnt to a crisp. A transmission fluid flush was carried out and new fluid was added. The family was soon back on the road fairly quickly and safely.
When it comes to Limp Mode, the reason that your car's onboard diagnostics (OBD II) detects a problem and triggers an error code that results in the Check Engine icon or the Limp Mode icon to appear can be caused by many different mechanical problems. Basically, your car triggers Limp Home Mode in order to protect the engine or transmission from further harm. It is not to be ignored.

Limp Mode lets you drive the vehicle a limited distance at a reduced speed so you can get it to a mechanic. Limp Mode usually limits the engine rpm and road speed. You'll notice a lack of power when you try to accelerate and low RPMs. Depending on the problem, it might limit the transmission to one gear and not let you engage a higher gear. In some cars, it can even shut down the air conditioner, depending on the error code. At least this mode will get you home or to a mechanic and not just leave you stranded somewhere. That's the entire idea for the invention of Limp Mode.

Sometimes, after the car sits for a while, Limp Mode will clear itself and you can drive again normally. But generally speaking, whatever problem caused Limp Mode to be activated will likely return, and often, with a vengeance.

Who Created Limp Mode?

Limp Mode was first invented by General Motors for use with OBD-I onboard computers in GM cars. Originally, it would be triggered when the ECM input voltage dropped below nine volts or the internal circuitry failed to ensure proper Computer Operating Pulses. In such conditions, the Limp Mode would revert to basic operating state. By 1992, the use of OBD-II diagnostics added features to allow drivers to get home such as code PO322 which is triggered by Knock Sensor Circuit Bank 2 Error or PO300, triggered by an engine misfire.

The car's computers are constantly monitoring engine performance. If the fuel and air mix is not correct, and it detects engine knock or pre-ignition, the car is programmed to try to protect itself. Sensors monitor the air coming into the engine, some sniff the exhaust emissions, and when sensors go bad, it can also trigger Limp Mode.
What to Do when the Limp Mode Light Comes On
If your car starts running rough and restricts performance, look for a safe place to pull off the road. If the Check Engine Light or Limp Mode indicator comes on, check your gauges for low fuel, oil pressure, coolant temperature, and battery charge. Also look for a transmission warning light. Check your Owner's Manual to find out what all the warning lights look like. Warning Lights are often different depending on the vehicle make and model.

You can check your oil level, brake fluid level, coolant level, and transmission fluid level yourself or leave it to a trained technician to handle the diagnosis and repair of your vehicle. Some vehicle owners who are mechanically inclined purchase an OBD II code reader to read the error codes. These scanners may be purchased from AutoZone, NAPA Auto, WalMart, Harbor Freight or Amazon for $20 to $50.

If you have a code reader, you can download the error codes which will give you a good indication of where the problem lies. You can also clear the codes and see if they pop up again. Sometimes cars throw random error codes and it's a one-off thing, but recurring error codes will tell you if there is a problem with a specific system.



Diagnosing the Problem

If you are checking over your car yourself, check the engine oil first. A lack of engine oil and low oil pressure can destroy your engine. Once you have pulled safely off the road, shut the engine off and give it a few minutes to cool down and for the oil to drain down into the oil pan before checking the oil level. Note that overfilling the oil can also cause problems. If the temperature gauge shows that the engine is running hot, you'll want to wait for the radiator to cool down before checking the coolant level. In that case, let the hood cool down before opening it as it will be hot as well.

Check the coolant overflow tank. If it is empty, wait about fifteen minutes for the radiator to cool down before opening the radiator cap to check the fluid level. If you open the hot radiator too soon, scalding steam or scorching hot water will pour out all over the place. If you don't have a container of coolant in the trunk, adding water will work.

Next, look to see if the serpentine belt is properly in place. One of its jobs is to keep the water pump turning. Without the serpentine belt, the water pump will not function to bring coolant to the engine, causing the engine to overheat. Likewise, that belt turns the car's alternator which keeps the battery charged.
If the serpentine belt is fine, check the car's hoses for leaks and check the engine air filter. A clogged air filter can also cause the engine to overheat. If the car is turbo-charged, sensors will expect a certain range of boost pressure from the turbo under certain driving conditions. If the sensors detect too much boost or not enough boost, it can trigger a Limp Mode condition. If a turbo pipe is split, leaking air and losing boost, it can also trigger Limp Mode. Listen for hissing air around the turbo connections.

Also look under the hood to see if any of the engine sensors have wires that are split or damaged from use. I once had mice in my engine bay. They chewed on some of the sensor wires and caused all kinds of error codes. Check sensor connections to make sure they are clean and firmly attached.
If you have a transmission warning light or the car will not shift properly in Limp Mode, check the transmission fluid level. Note that many of today's cars do not allow you to check the transmission fluid level. If that is the case, have a factory-trained technician check the transmission for you.

With that in mind, if your car goes into Limp Mode, it is best to drive the car directly to an authorized service center and have technicians check the error codes to diagnose the problem. Even if the car's computer cleared Limp Mode while you were checking things over, you can bet that the problem will show up again. 
Let Lithia Help
Lithia Motors has a service center near you that can clear those error codes and find out what triggered Limp Mode in the first place. With over 75 years of experience servicing cars of ever make and model, we have the trained professionals, expertise and equipment to get you back on the road.