4A-GE 20V Enthusiast

Oxygen sensors have been standard equipment on passenger car and light truck engines since 1980-81/ Most such vehicles have one or two oxygen sensors (two are typically used on selected V6 and V8 engines starting in the late 80s). Since the introduction of Onboard Diagnostics II (OBD II) in 1995-96, the number of oxygen sensors per vehicle has doubled (the extra sensors are used downstream of the catalytic converter to monitor its operating efficiency). Yet, as important as oxygen sensors are today, few motorists are even aware of their presence - let alone the key role oxygen sensors play in engine performance and reducing pollution. One survey found that 99.7% of all consumers did not know their vehicle had an oxygen sensor .

What does O2 sensor do ?

It is the primary measurement device for the fuel control computer in your car to know if the engine is too rich or too lean. The O2 sensor is active anytime it is hot enough, but the computer only uses this information in the closed loop mode. Closed loop is the operating mode where all engine control sensors including the Oxygen sensor are used to get best fuel economy, lowest emissions, and good power.

How O2 sensor fights pollution

Originally called a "Lambda sensor" when it was first used in fuel-injection European applications, the oxygen sensor monitors the level of oxygen (O2) in the exhaust so an onboard computer can regulate the air/fuel mixture to reduce emissions. The sensor is mounted in the exhaust manifold and generates a voltage signal proportional to the amount of oxygen in the exhaust. The sensing element on 99% of all oxygen sensors in use is a zirconium ceramic bulb coated on both sides with a thin layer of platinum. The outside of the bulb is exposed to the hot exhaust gases, while the inside of the bulb is vented internally through the sensor body or wiring to the outside atmosphere.

When the air/fuel mixture is rich and there is little O2 in the exhaust, the difference in oxygen levels across the sensing element generates a voltage through the sensor’s platinum electrodes: typically 0.8 to 0.9 volts. When the air/fuel mixture is lean and there is more oxygen in the exhaust, the sensor’s voltage drops to 0.1 to 0.3 volts. When the air/fuel mixture is perfectly balanced and combustion is cleanest, the sensor’s output voltage is around 0.45 volts.

The oxygen sensor’s voltage signal is monitored by the onboard engine management computer to regulate the fuel mixture. When the computer sees a rich signal (high voltage) from the O2 sensor, it commands the fuel mixture to go lean. When the computer receives a lean signal (low voltage) from the O2 sensor, it commands the fuel mixture to go rich. Cycling back and forth from rich to lean averages out the overall air/fuel mixture to minimize emissions and to help the catalytic converter operate at peak efficiency (which is necessary to reduce hydrocarbon (HC), carbon monoxide (CO) and oxides of nitrogen (NOX) levels even further).

The speed with which the oxygen sensor reacts to oxygen changes in the exhaust is very important for accurate fuel control, peak fuel economy, and low emissions. The air/fuel mixture in an older carbureted engine doesn’t change as quickly as that in a throttle body fuel-injected application, so response time is less critical. But, in newer engines with multipoint fuel injection, the air/fuel mixture can change extremely fast, requiring a very quick response from the o2 sensor.

Sensor aging

Nothing is lasts forever, O2 sensor are no execption. Time period of O2 sensor are about 30-60,000 km or 2000 key start. Mostly after that a reminder from engine check light will appear. Consult your owner manual, auto repair manual, dealer, or repair shop on what your engine lights means. (For 4AGE 20V the error code is 21).

Contiminants from normal conbustion and oil ash accumulate on the sensing element. This reduces This reduces the sensor ’s ability to respond quickly to changes in the air/fuel mixture. The sensor slows down and becomes "sluggish." At the same time, the sensor’s output voltage may not be as high as it once was, giving the false impression that the air/fuel mixture is leaner than it actually is. The result can be a richer-than-normal air/fuel mixture under various operating conditions that causes fuel consumption and emissions to rise.

How do I know if my O2 sensor may be bad

These *is not* a pointer to O2 sensor failure, just bring up the possibility. Vacuum leaks and ignition problems are the common fuel economy killer.
i)  Fuel consumption increase
ii) Engine will lost power and not rspond quickly although done the normal tuning
iii) Gas from the exhaust smell like rotten egg
iv) No more water steam look alike came out from the exhasut tip (bad engine condition also happen like this)

How can I test my O2 sensor ?

They can be tested both in the car and out. If you have a high impedence volt meter, the procedure is fairly simple. It will help you to have some background on the way the sensor does it’s job. Read how does an O2 sensor work first.

Testing O2 sensor that are installed

The engine must first be fully warm. If you have a defective thermostat, this test may not be possible due to a minimum temperature required for closed loop operation. Attach the positive lead of a high impedence DC voltmeter to the Oxygen sensor output wire. This wire should remain attached to the computer. You will have to back probe the connection or use a jumper wire to get access. The negative lead should be attached to a good clean ground on the engine block or accessory bracket. Cheap voltmeters will not give accurate results because they load down the circuit and absorb the voltage that they are attempting to measure. A acceptable value is 1,000,000 ohms/volt or more on the DC voltage. Most (if not all) digital voltmeters meet this need. Few (if any) non-powered analog (needle style) voltmeters do. Check the specs for your meter to find out. Set your meter to look for 1 volt DC. Many late model cars use a heated O2 sensor. These have either two or three wires instead of one. Heated sensors will have 12 volts on one lead, ground on the other, and the sensor signal on the third. If you have two or three wires, use a 15 or higher volt scale on the meter until you know which is the sensor output wire.

When you turn the key on, do not start the engine. You should see a change in voltage on the meter in most late model cars. If not, check your connections. Next, check your leads to make sure you won’t wrap up any wires in the belts, etc. then start the engine. You should run the engine above 2000 rpm for two minutes to warm the O2 sensor and try to get into closed loop. Closed loop operation is indicated by the sensor showing several cross counts per second. It may help to rev the engine between idle and about 3000 rpm several times.

There are two types of knock sensors used on Toyota engines.

i) The mass type sensor, which produces a voltage output over a wide input frequency range (vibration frequency is about 7KHz). The ECU uses a filter circuit to identify between background noise and actual engine knock.

ii) The resonance sensor, is tuned into a very narrow frequency band and only produces a certain signal voltage when exposed to vibration in the range of 7KHz. The ECU requires less complicated filter curcuit with this type of sensor.

ECU Detonation Control

Below is how Knock sensor works with ECU…

when there’s a detonation taking place, it retards ignition timing until the knocking stop. The the timing advanced back to normal or calculated value. Due to this ignition system will operates at maximum performance. If the ECU noted that the knock retard is not workingm it will enter a fail-safe mode and fix the retard angle to prevent engine damage.

Symptoms

Drivability problems including a lack of power and perhaps some hesitation also increase of fuel consumption. There may also be other symptoms.

i) The knock sensor is faulty and need to be replaced
ii) There is a wiring shoft/failure in the knock sensor circuit. Check properly because sometimes after change the knock sensor problem or error code still shows…
iii) Also check for special ground shield that cover the signal wire running from the sensor to the sensor. The shield sourrounds the signal wire is connected to ground so if any electrical interference is taken to ground.

sometimes the engine light won’t come out, therefore no error code were found, but some say that they just replace the knock sensor thier car feel like new, in other hand they replace the knock sensor there’s not much different, later they found out that the wiring was faulty…so becarefull…

i) By removing the knock sensor, check if there is no continuity between the terminal and body (based on 4A-GE repair manual, page F1-67)
ii) checking the diagnosis code

Sometimes a knock sensor will react to sounds other than those produced by detonation. A noisy mechanical fuel pump, a bad water pump or alternator bearing, or a loose rod bearing can all produce vibrations that can trick a knock sensor into retarding timing.