Is Your Client’s Car in Limp Mode? A Brief Guide for Anyone Taking a Mechanic Program

The check engine light starts flashing, and a vehicle refuses to speed up even when flooring the gas pedal. What’s happening? Most likely, these are signs that a vehicle has gone into limp mode. 

Limp mode, also often referred to as “limp home mode,” is a protective feature programmed into the engine control unit or transmission control unit of most vehicles. If a vehicle’s internal system senses a problem within a powertrain component, it may activate limp mode in order to prevent further damage to the engine. Limp mode allows drivers to “limp” to a mechanic in order to properly diagnose and fix the problem that’s occurring, reducing the power and controlling the RPM in order to keep the vehicle and its occupants safe. 

If you’re considering a career as an auto mechanic, your services might be required in order to identify the symptoms and causes of limp mode and make the necessary repairs. Read on to find out more about this critical feature.

Common Symptoms of Limp Mode Explained for Those in Automotive School 

There are a few ways to tell whether a vehicle has entered limp mode. One of the most telling indicators is when a vehicle’s speed and RPM are limited. Limp mode’s primary function is to limit the power of a vehicle’s engine and transmission, and when activated, a vehicle’s RPM will not go over 3,000 while its speed stays below 65 km/h. Another common symptom is a flashing ‘Check Engine’ light, which is often the first thing that will catch a driver’s attention upon entering limp mode. 

Limp mode can often affect a vehicle’s performance and transmission. If a driver notices that their vehicle won’t accelerate, or is misfiring when attempting to speed up, limp mode may have been activated. Additionally, a vehicle that’s entered this mode often won’t shift over third gear—an automated means of protecting the engine. When a driver experiences a few of these symptoms, it’s time for them to take their car in to a garage to be examined by a mechanic program graduate. 

Possible Causes of Limp Mode

While limp mode can manifest in many different ways, once activated, it’s important to determine the cause behind it. There are a few common causes. For one, low fluid levels can activate this mode, especially when the transmission fluid or engine oil is low. Low fluid levels can decrease oil pressure, potentially causing a vehicle to enter limp mode. 

In turbocharged vehicles, boost-control problems are a common catalyst for this automotive mode. Boost-control problems might include a boost leak or an over-boost, which happens when the boost pressure is higher than the engine control unit’s target boost pressure. In this case, limp mode may be activated in order to prevent engine damage.

Limp mode can also be triggered by a malfunctioning sensor. MAF, MAP, speed, or TPS sensors can all activate limp mode if they aren’t working properly, as the engine control unit will become confused by malfunctioning signals. Another cause of limp mode may be broken or damaged wiring within a vehicle. If a connection is corroded or a wire is harmed by debris or heat, it may not be able to transmit signals to the engine or transmission, causing the engine control unit to activate limp mode as a precaution. If you’re currently enrolled in automotive school, it’s important to know to check for these possible causes when diagnosing a vehicle in limp mode.

Fixing Limp Mode

When a vehicle enters limp mode, it’s important to recognize that it’s a preventative measure, designed to protect a vehicle from further damage to its powertrain. Thus, a driver should pull over and arrange to take their vehicle to the nearest mechanic when this feature has been activated. When a vehicle arrives to a garage in limp mode, an auto service technician can use an OBD2 scanner in order to check the trouble codes stored in the computer. Unless a wire is sending the wrong signal to a vehicle’s computer, trouble codes can be used in order to identify which parts need replacing. 

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