Exhaust Pressure and Lambda Probe Testers

Frequently Asked Questions About Exhaust Pressure and Lambda Sensor Testing

Elevated exhaust backpressure or abnormal lambda control can have various causes and cannot always be assessed solely through electronic vehicle diagnostics. Mechanical tests provide additional information about the actual condition of the exhaust system. Here, we answer frequently asked questions about exhaust pressure measurement and functional testing of lambda sensors.

• What is exhaust pressure or exhaust back pressure?

  Exhaust back pressure is the pressure the engine must overcome so that the combustion exhaust gases can flow through the exhaust system and aftertreatment system.

  A certain amount of flow resistance is inherent in the system.

  However, if the backpressure becomes unusually high, this may indicate a restriction in the exhaust path.

  This pressure can be measured mechanically using a suitable exhaust pressure tester.

• Why is exhaust backpressure measured?

  Measuring exhaust pressure may be useful in the following cases:

  • Loss of power
  • Poor engine startup
  • Unusual turbocharger behavior
  • Suspected restricted exhaust flow
  • Suspected clogged or damaged catalytic converter
  • Inconsistent differential pressure readings

  This measurement helps determine whether there is increased flow resistance in the exhaust system.

• What are the possible causes of increased exhaust back pressure?

  Possible causes depend on the specific vehicle and exhaust system.

  Possible causes include, for example:

  • a restricted catalytic converter
  • a clogged exhaust aftertreatment system
  • a damaged or deformed exhaust pipe
  • internal damage to a muffler or catalytic converter
  • other mechanical restrictions in the exhaust path

  The measured pressure must be evaluated in accordance with the manufacturer’s specifications.

• What symptoms can high exhaust backpressure cause?

  Possible symptoms include, for example:

  • loss of power
  • delayed throttle response
  • reduced engine power under load
  • abnormal boost pressure behavior
  • increased thermal stress
  • unusual engine or exhaust emissions

  However, these symptoms are not definitive.

  An actual pressure measurement helps to confirm or rule out the exhaust system as a possible cause of the problem.

• How is exhaust pressure measured using the AGD 01?

  The AGD 01 is connected to the exhaust system via a suitable measuring point.
  
  In the specified test procedure, the connection between the engine and the catalytic converter is made using the supplied connector instead of the lambda sensor.
  
  The actual exhaust pressure can then be read on the pressure gauge while the engine is running.
  
  The evaluation must be performed in accordance with the manufacturer’s specifications or the specified test procedure.

• What is the measurement range of the AGD 01?

  The Leitenberger AGD 01 has a measurement range of -1 to +3 bar.
  
  It comes with a 3-meter-long pressure hose and an M18x1.5 connector.
  
  This allows the measured value to be clearly displayed within the mechanic’s work area during the intended test.

• Does high exhaust pressure automatically mean that the catalytic converter is defective?

  No.
  
  High exhaust backpressure initially indicates that there is unusual flow resistance in the exhaust system.
  
  The exact cause must then be identified.
  
  The catalytic converter may be a possible cause, but other components of the exhaust system can also restrict flow.
  
  Therefore, a red/green indicator on the test device should be viewed as a diagnostic aid and not as sole evidence of damage to a specific component.

• What is the difference between exhaust backpressure and differential pressure at the particulate filter?

  Exhaust backpressure measures the actual pressure at a defined point in the exhaust system relative to ambient pressure.
  
  A differential pressure sensor, on the other hand, measures the pressure difference between two measurement points.
  
  Both measurements can provide indications of flow resistance, but they are not identical.
  
  If electronic measurements are implausible, an independent mechanical pressure test can provide additional information.

• Why can exhaust back pressure be relevant for turbocharger diagnostics?

  In a turbocharged engine, the exhaust gas flows through the turbine side of the turbocharger.

  An unusual restriction in the downstream exhaust system can therefore affect the operating behavior of the engine and turbocharger.

  If such symptoms are present, in addition to measuring exhaust gas pressure, a more comprehensive turbocharger diagnosis, including measurements of boost pressure and crankcase pressure, may be advisable.

• What does a lambda sensor do?

  A lambda sensor measures the oxygen content—or a parameter derived from it—in the exhaust gas.
  
  The engine management system uses the signal to monitor and control the combustion process or the exhaust aftertreatment system.
  
  The specific function of a given sensor depends on the vehicle, the engine management system, and the sensor type.

• Why is a lambda sensor tested?

  An electronic error code does not always clearly indicate whether the cause is the lambda sensor itself, the air-fuel mixture, or some other factor.
  
  In compatible systems, therefore, the lambda sensor’s response behavior can be specifically tested.
  
  This helps determine whether the sensor responds to a defined change in operating conditions.

• What is a disturbance valve?

  A disturbance valve makes it possible to supply the system with a precisely metered additional volume of air.
  
  In the Leitenberger SV 01, this disturbance air is introduced via the vacuum line upstream of the activated carbon filter.
  
  This allows you to verify, in systems designed for this purpose, how the lambda sensor or the air-fuel mixture control system reacts to the defined change.

• How does the lambda sensor test work with the SV 01?

  The SV 01 is installed in the vacuum line upstream of the activated carbon filter in accordance with the specified test procedure.
  
  The adjustable valve is used to turn a defined amount of interference air on or off.
  
  It is then possible to observe whether and how the lambda control system reacts to this change.
  
  The SV 01 is adjustable from 0.0 to 2.3 mm.

• Is the SV 01 an electronic lambda sensor tester?

  No.
  
  The SV 01 does not directly measure the electrical output signal of the lambda sensor.
  
  It generates a defined disturbance signal that can be used to test the system’s response or control behavior.
  
  Additional measuring or diagnostic devices are required for electrical testing of voltage, signals, or wiring.

• Can a leak in the exhaust system affect the lambda sensor signal?

  Yes.
  
  If additional ambient air enters the exhaust stream at an unfavorable location, the oxygen content at the measurement point may change.
  
  This can affect lambda sensor readings or control values.
  
  If measurement results are implausible, the mechanical condition of the exhaust system should therefore be considered in addition to the sensor, wiring, and air-fuel mixture.

• Is this subcategory suitable for a leak test of the exhaust duct?

  The devices currently available in this subcategory are primarily intended for measuring exhaust pressure and testing the functionality of lambda sensors.
  
  A complete leak detection or leak test of the exhaust system may require a different testing method and appropriately suited testing equipment.
  
  Therefore, a distinction should be made between exhaust pressure testing, lambda sensor testing, and actual leak detection.

• What other tests are recommended if there are exhaust problems?

  Depending on the fault symptoms, the following may also be useful:

  • Inspection of SCR and AdBlue® systems
  • Turbocharger diagnostics
  • Electronic vehicle diagnostics
  • Inspection of the intake and charge air systems
  • Inspection of exhaust sensors and wiring

  For additional mechanical diagnostic solutions, see Test and Diagnostic Equipment for Exhaust Systems.