Electronic throttle: limp mode, cleaning and relearn

On almost every modern gasoline engine there is no longer a cable between the pedal and the engine. The pedal sends two signals to the computer, which decides how far to open the throttle plate and moves it with a small electric motor. The system is reliable, but when something does not add up the computer chooses to protect the driver and leaves the engine with almost no power. That is the well-known limp mode.

How to recognize limp mode

• The engine revs up very slowly or will not go past about 2,000 to 3,000 rpm.
• The check engine light comes on and, on some vehicles, a warning light with a throttle or wrench symbol.
• The idle may stay high or very low, and the engine may stall when you stop.

The usual codes are P2135 and P2138 (sensor signals that do not agree), P2101, P2111 and P2112 (the throttle does not follow its command), P2119 and P2104 to P2110 (protection modes) and P2176 (idle position not learned).

The most common causes

1. Dirt in the throttle body. Oil vapors from the crankcase ventilation leave a coating that holds the plate back from its rest position.
2. Corroded connectors on the throttle or the pedal. A small resistance shifts one of the two signals and the sensors stop agreeing.
3. A weak battery or loose grounds. With low voltage, the throttle motor does not respond in time.
4. Worn sensors in the pedal or the throttle body, which usually come built into the assembly.
5. A worn throttle motor or gears on high-mileage vehicles.

How to clean the throttle body

1. Switch the ignition off and wait a minute. Never move the throttle plate by hand with the ignition on: the computer may learn the wrong position.
2. Remove the intake duct that leads to the throttle body.
3. Apply carburetor or throttle body cleaner to a cloth, not directly onto the electric motor or the connector.
4. Clean the edge of the plate and the inside of the bore, opening the plate gently with a finger. If the manufacturer forbids this, remove the throttle body to clean it.
5. Let the cleaner evaporate, refit the duct and run the relearn.

The idle relearn

After cleaning the throttle body, disconnecting the battery or replacing the part, the computer needs to learn the rest position again. Many scan tools have a throttle relearn or reset function. Without a scan tool, some manufacturers let you do it by switching the ignition on for a few seconds without starting, then starting the engine and letting it idle for several minutes without touching the pedal or switching on any loads. The procedure varies between makes: check your vehicle's manual.

When cleaning is not enough

If the codes return after cleaning and relearning, graph the two pedal signals and the two throttle signals on the scan tool while you press the pedal slowly. They should move smoothly and in proportion. A jump or a dropout in one of them points to a worn sensor, and the fix is to replace the whole pedal or throttle body.