Air flow meter (hot wire/film - petrol)

The purpose of this test is to evaluate the voltage output and response time of the Air Flow Meter (AFM) during engine idle, Wide Open Throttle (WOT) and over-run conditions. 

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

  1.  Use the vehicle wiring diagram to identify the signal circuit.
  2. Connect to Channel A of your PicoScope.
  3. Start the engine.
  4. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  5. Start the scope to see live data.
  6. Carry out a WOT test to capture your waveform. 
  7. With your live waveform on screen stop the scope.
  8. Turn off the engine.
  9. Use the Waveform Buffer, Zoom and Measuring tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

With the engine idling, the initial voltage is around 0.5 V.

When the accelerator pedal is rapidly and fully depressed, the voltage rises rapidly to almost 4.0 V, then falls to around 2.5 V. This initial voltage peak is due to the inrush of filtered air into the manifold when the throttle becomes fully open.

When the manifold is filled and its air pressure is equal to the ambient air pressure, the subsequent airflow is dependent on the engine speed. Therefore, as the engine speed increases to its peak, the voltage rises to around 4 V.

When the accelerator pedal is released and the throttle closes, the air intake and signal voltage drop rapidly.

As the engine overruns and its speed drops, the sensor voltage gradually reduces as the engine returns to its idle state. Some engine control modules will maintain or even increase airflow in this phase as an anti-stall feature.

The waveform hash is due to fluctuations caused by engine induction pulses.

Waveform Library

Go to the drop-down menu bar in the lower left corner of the Waveform Library window and select Mass air flow sensor (MAF) hot wire.

Example Hot wire AFM

Further guidance

Air flow meters measure the quantity of filtered air entering an engine. As such, they are used by the Engine Control Module (ECM) as the primary engine load sensor.

Hot wire air flow meters have a heated wire element located within the intake air flow. The voltage, and hence current, through the hot-wire circuit is varied to keep it at a fixed, hot, temperature. As the air flow increases, its cooling effect increases and the greater the voltage required to keep the wire temperature constant. Therefore, the hot-wire circuit voltage indicates the air flow to the ECM. The sensor element and air flow meter body form a calibrated unit and are not interchangeable.

Due to their position within the air intake tract, air flow meters can be prone to contamination, e.g. if a vehicle is poorly serviced with ineffectual air filters, or if it has other intake, crankcase breather, or exhaust recirculation faults. In such cases, the meter’s readings can be erroneous, causing the ECM to incorrectly calculate the air-fuel mixture, possibly leading to poor performance, incomplete combustion events and other fuel trim or mixture related faults.

Note: Some manufacturers use hot film air flow meters. The testing procedure and operating characteristics are the same as the hot wire type.

Diagnostic trouble codes

Selection of component-related Diagnostic Trouble Codes (DTCs):

P00BC Mass or Volume Air Flow "A" Circuit Range/Performance - Air Flow Too Low

P00BD Mass or Volume Air Flow "A" Circuit Range/Performance - Air Flow Too High

P00BE Mass or Volume Air Flow "B" Circuit Range/Performance - Air Flow Too Low

P00BF Mass or Volume Air Flow "B" Circuit Range/Performance - Air Flow Too High

P0100 Mass or Volume Air Flow "A" Circuit Malfunction

P0101 Mass or Volume Air Flow "A" Circuit Range/Performance Problem

P0102 Mass or Volume Air Flow "A" Circuit Low Input

P0103 Mass or Volume Air Flow "A" Circuit High Input

P0104 Mass or Volume Air Flow "A" Circuit Intermittent

P010A Mass or Volume Air Flow "B" Circuit

P010B Mass or Volume Air Flow "B" Circuit Range/Performance

P010C Mass or Volume Air Flow "B" Circuit Low

P010D Mass or Volume Air Flow "B" Circuit High

P010E Mass or Volume Air Flow "B" Circuit Intermittent/Erratic


This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • Premium 6-way breakout lead set


  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • Large Dolphin/Gator Clips


  • Premium Test Lead: BNC to 4 mm, 3 m


  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)


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Guided test: Air flow meter (hot wire/film - petrol)