Toyota RAV4 Forums banner

1 - 5 of 5 Posts

·
Registered
Joined
·
69 Posts
Discussion Starter #1
Most early year's 4 cylinder fuel inject cars are equipped with an oxygen sensor to realize close loop control. OBDII requires at least 2 oxygen sensors in order to detect status of a catalytic converter. From posts on topic of "Check engine light is ON - dang!", I notice that 2001 and later RAV4 come with 2 oxygen sensors plus 2 A/F sensors.

The A/F sensor is basically a newly developed oxygen sensor. It plays exactly the same role as the oxygen sensor, although it is constructed differently. The A/F sensor is obviously better than the oxygen sensor because of wide range, linear output, .... These advantages make ECM to control a vehicle more accurately.

Why does Toyota use the A/F and oxygen sensors parallelly? Is it because higher operating temperature (750C) of the A/F sensors, so let the low temperature (400C) oxygen sensors monitor air fuel ratio before an engine fully warms up? Or the A/F sensors are not reliable, therefore use the oxygen sensors as backup?
 

·
Registered
Joined
·
986 Posts
It's called a feedback loop.

The two A/F sensors are placed before the cat, and the two O2 sensors are placed after the cat. As you said, the two sensors are designed differently; the A/F sensors are specifically designed to work with the exhaust gasses before they are converted, and the O2 senors are designed to work with the exhaust gasses after they are converted.

When the engine is running, the air/fuel mixture goes back and forth between rich and lean. The two A/F sensors monitor these variations in the exhaust mixture before the gasses go into the cat. The two O2 sensors measure the mixture after the cat, and the ECU compares the readings from both sets of sensors. Essentially what happens is that when the mixture entering the cat shifts between rich and lean, if the cat converter is working properly, the exhaust coming out of the cat will also shift between rich and lean. What the ECU does is compare the readings between the two sets of sensors to determine if the cat is working efficiently. If you were to look at a graph of the readings from the sensors, the readings of the O2 sensors lag behind the readings of the A/F sensors by a certain rate. If the readings from all the sensors were shifting at the same rate (or not shifting in a sequence), then there is a problem with the cat.
 

·
Registered
Joined
·
69 Posts
Discussion Starter #3
I have no electronic component location diagram for 2000 and later RAV4. I guessed one A/F and oxygen sensors are in front of cat converter, another A/F and oxygen sensors locate at rear of the cat converter. Now the fact is that 2 A/F sensors are in front and 2 oxygen sensors are at rear.
It confuses me more. It is a 4 cylinder engine with only one exhaust pipe. To leave a pair of the same character sensors in the same location is obviously redundant.
 

·
Registered
Joined
·
986 Posts
The 4.2 Ravs have dual cats integrated into the exhaust header in front of the engine under the heat shield. 2 cats = 2 banks of sensors. Why they have 2 cats I have no idea.
 

·
Registered
Joined
·
69 Posts
Discussion Starter #5
Configuration of A/F, oxygen and cat converter in California vehicle or an automatic transmission vehecle is often different from other states or manual transmission cars. Is this 2 A/F + 2 oxygen sensors layout the same in all 4.2 RAV4?
 
1 - 5 of 5 Posts
Top