Limp home mode sensor diagnosis driveability, check engine

Auto Computer Controls

Modern automobiles (from the 1980's to present) have at least one computer controlling them. This computer (or computers) control the fuel to air mixture via either electronic fuel injection or a feedback carburetor. On the newer vehicles the computer also controls the ignition timing (That's when the spark plugs fire).

The newer a vehicle is,the more stuff the computer(s) control. The newest vehicles have the computer shifting the automatic transmission or transaxle as well as computer controlled spark and fuel mix. On some vehicles the throttle is controlled by a servo motor driven by the computer. With this "drive by wire" throttle system there is no direct link from the pedal to the throttle! Many vehicles also have a climate control computer which controls the A/C and heat. Luxury cars often have vehicle anti-theft controlled by computer: some even will adjust the seat and steering wheel to suit different drivers when they enter the car!!!

Disadvantages of Computer Controls

The downside to all this computer "smarts" on a car is obvious: what happens when something breaks? The good thing is that the onboard computer itself is very reliable. The problem is that the computer relies on a whole flock of sensors and wires to give them the data needed to make your car run. These sensors are not nearly as reliable as the computer itself, in fact they fail quite often!

Limp Home Mode

This is where a "limp home mode" comes into play. Whenever the vehicle computer gets a reading from a sensor that is obviously wrong, it will "assume" a value that it "knows will work". On the earliest computer controlled cars with feedback carburetors, if the computer got bad readings from sensors, it would run the carb at the richest setting. You would get horrible gas mileage, but the car WOULD run. The computer would then turn on an amber CHECK ENGINE LIGHT It would also store a trouble code in its memory telling what sensor was giving a "bogus" reading.

A newer car computer is much more sophisticated, and just because a "CHECK ENGINE" light comes on doesn't really mean the computer is totally in a "limp home mode". Depending on what sensor reading is out of specs the computer may still be doing a pretty good job of controlling the engine. For example, some cars can have the "CHECK ENGINE" light come on if you don't tighten your gas cap tight enough after filling up!

On most vehicles the CHECK ENGINE LIGHT will go out if the sensor starts giving normal readings again, although some computers will make you use a scan tool to turn the light off.

Here are some of the many MIL (malfunction indicator light) designs

Click here to see more images of CHECK ENGINE or MIL (malfunction indicator) lights

Computer Diagnostic Trouble Codes (DTC's)

If the MIL (Check Engine) light is on, a Diagnostic Trouble Code has been recorded in the computer, even if the light goes off after awhile. These codes can be recalled even after the engine has been turned off. These computer codes tell which sensor has given an "out of specs" reading. Some cars will "read out" their trouble codes by blinking the MIL (Check Engine) light after grounding an electric terminal under the dash, or under the hood. On older Chrysler products cycling the key switch off and on 3 times would make the computer "blink" it's trouble codes. (This may still work on newer Chryslers!) The trouble codes can be erased from an older computer using a scan tool, or by disconnecting the battery or pulling the computer fuse for a few minutes. Some newer computer systems keep the codes in memory until they are reset by a scan tool. Click here for more info on reading trouble codes

Intermittent computer sensor problems

Sometimes a check engine light will go off after awhile: this means the computer sensor is reading "OK" again and the computer is happy. An intermittent sensor problem on most vehicles still causes a trouble code to be stored in the computer.

Antilock Brake and Airbag Computer

The antilock brake system and the airbag system both have their own computer. They MAY communicate with each other on SOME vehicles, but are capable of doing their jobs without input from the other vehicle computers. The antilock brake computer and the airbag computer have their own "MIL" type lights: the ABS one says "ABS" and the antilock brake light usually says "antilock". These lights are normally amber.

COMPUTER CONTROLLED TRANSMISSION LIMP HOME MODE

Most of the time a computer in limp home mode will just turn on the CHECK ENGINE light and give you bad performance and fuel economy. A big exception to this is on computer shifted transmissions. With these cars (Chryslers especially) if the shift control computer gets a bad sensor value, or if it detects a problem with the transmission, it will put the transmission into a VERY obvious limp home mode!!! You will have a normal reverse gear, but in any forward range you will have second gear only. No 1st, no 3rd or 4th. Just second gear. This, of course, limits your top speed to around 45 to 50 MPH. Quite annoying, but it beats walking! It allows you to "limp home" and get the car to a shop to find out what is wrong.

COMPUTER CONTROLLED THROTTLE BODY LIMP HOME MODE

Many newer vehicles have a "drive by wire" throttle. This means there is no cable or linkage directly linking the accelerator pedal to the throttle.

(Note: some systems have a "bypass" that "creates" a physical link to the throttle if the computer control fails. (Click here to see a system like that on a SAAB)

The manufacturer goes to great lengths to program electronic throttle control to be just like the old style throttle as far as the driver is concerned.

The limp home mode on electronic throttle control WITHOUT a mechanical backup system like the SAAB above, is to hold the engine at a high idle. This idle will be high enough to "limp home" at 30-40 MPH, but obviously can't be driven much before repair.

Older vehicle engine limp home modes

Actually the computer has had "it's foot on the gas pedal" for quite some time. The idle speed has been computer controlled on all fuel injected vehicles since the early 80's. Even before that the A/C had a system (analog) to idle the car up when the compressor went on.

A high idle on any vehicle less than 30 years old is probably due to an incorrect sensor reading (like throttle position sensor). The computer has gone into a "limp home mode" where it idles the engine up super high. There will be a check engine, or MIL light, unless it's burned out! This means the computer has stored a trouble code to help diagnose the problem.

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Thank you for visiting the ECONOMECHANIX WEB SITE. Please feel free to comment. We also serve the surrounding communities of Alachua, High Springs, Hawthorne, and Newberry! Gainesville has been my home since 1974, and I've loved Gvl and the Gators since I came here in the fall of 1974 to attend the University of Florida. I loved it so much I stayed and opened my car repair business. Originally it was out of the back of a 1963 Chevrolet wagon, but in 1977 a fellow mechanic and I opened an auto repair shop with actual walls, etc. I stayed in the same location for 26 years, and recently moved my operation to property I bought 15 miles east of Gainesville. I am doing most all the repairs myself now, having reduced my overhead from $1500 per month to practically nothing. I do work by appointment only. I mostly work only on my established customers cars, but I will occasionally take on new clients. E-mail me and I will either make arrangements to look at your car, or I will recommend you to someone who will.

George G. Scott, Jr.