The lambda sensor "Open Loop" or "Closed Loop" @

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It is incorrect to assume that the A/F mixture is "Rich" or "Lean" from the Lambda sensor voltage. The Lambda sensor signal only reports higher or lower oxygen content in the exhaust gases. Nothing else.

Operating principle of a "Control Lambda Sensor"

The "Lambda sensor" (is an oxygen sensor in the exhaust system) is a basic and most important component in controlling the A/F mixture on petrol engines. Integrating a Lambda sensor (designed by BOSCH in 1976) into the exhaust system before the three-way catalytic converter represents a major step since 1989 in the development of emission systems. From that time the Lambda sensor has taken over complete "feedback control" responsibility for adjusting A/F mixture values. This has enabled to cancel all mechanical adjustments of the A/F mixture.

The oxygen sensor status information provided by OBD diagnostics has high importance.

What is "Open Loop"? 

The "OPEN LOOP" status is signalling that the control unit is not using the Lambda sensor signals to calculate the A/F mixture:
A) after cold start when the Lambda sensor has not reached its working temperature
B) when there is a fault in the electrical circuit of the Lambda sensor

What is "Closed Loop"?

The "CLOSED LOOP" status is signalling that the control unit is using the Lamda sensor signals to calculate the A/F mixture (normal status when operating temperature has been reached).

"Closed loop" feedback verifies the A/F mixture

If the oxygen content in the exhaust emissions is not correct the control unit responds and adjusts the mixture so that the oxygen content returns to the "Lambda window". Detailed information about mixture control can be found in article "Mixture adaptation".

DTC

• Lambda sensor heating - short / open 
• Lambda sensor - implausible signal ... etc.

As we can see from the DTCs, they are only straighforward and indicate a problem with mixture control. The technician must know all possible causes (described in "Mixture control problems") and determine the actual fault cause.

Mixture control diagnostics

• Establish if the Lambda sensor is in "CLOSED LOOP" mode
• Read the "Fuel mixture adaptive" values (EOBD - Long Term Fuel Trim)
• With an oscilloscope measure the Lambda sensor ground signals
• Disconnect the Lambda sensor (simulation)
• Measure the exhaust tail pipe emissions with an analyzer
• Verify the ignition signals and all other parameters influencing the A/F mixture

Tests

1. If the step Zirconia Lambda sensor has a short signal problem the control unit switches...
1. ..to CLOSED LOOP mode
2. ..to OPEN LOOP mode
3. ..if the short is confirmed a DTC is set and the mode is switched to CLOSED LOOP
   
   
2. If the Zirconia step Lambda sensor is sending to the control unit a voltage signal with a value around 1 V...
1. ..it means that the A/F mixture is rich.
2. ..it means that the A/F mixture is lean.
3. ..it means that residual oxygen content in the exhaust emissions is reduced.
   
   
3. What is the signal voltage response of the step Zirconia Lambda sensor when the EGR valve is leaking?
1. The Lambda sensor voltage is going to be low.
2. The Lambda sensor voltage is going to be high.
3. No response because the EGR system has no relation to the residue oxygen content in the exhust emissions.

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Correctly working "Closed Loop" mode with 2 control Lambda sensors (1732/11)

Recording of 2 control Lambda sensors from a 6-cylinder Honda Acura NSX engine, where the control unit based on feedback information is regulating the A/F mixture so that it is within a so called "Lambda window" (0.97 to 1.03). Inside these values, the 3-way catalytic converter has the highest efficiency (3-way CO, HC, NOx).

Closed Loop mode at full load (1732/12)

The Lambda sensor signal doesn't always have to alternate inside the "Lambda window". The signals of these two control Lambda sensors have been recorded during acceleration of a V6 Honda NSX engine. The control unit adjusts a "rich" A/F mixture (Lambda value 0,89 = is out of the "Lambda window"), where the Lambda sensor voltage is at its max, range limit and it doesn't alternate.

Closed Loop mode - misfire on one bank (1732/13)

Full load where one cylinder on one cylinder bank of the V6 Honda NSX engines misfires. Here we have good evidence of quick response of the oxygen sensor (Lambda sensor) to the oxygen shocks coming from misfire of one spark plug from the left cylinder bank. This distorted signal due to misfire is a command for the control unit to apply unjustified A/F mixture enrichment that worsens the condition to a point where all the spark plugs become flooded.

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