ELM327 MAF Check

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An OBD-II test tool that compares the air your MAF sensor measures with the air the engine should actually be taking in. Works with an ELM327 adapter (Bluetooth or WiFi) and supports petrol and diesel engines.

What does MAF Checker do?

The MAF sensor measures the air entering the engine, and the ECU largely calculates the fuel quantity from that value. A dirty or ageing MAF under-reads; the engine hesitates, loses pulling power, and fuel consumption and smoke increase. The catch is that a “wrong” MAF is hard to spot in live data: you only see a number, and you don’t know what that number should be.

MAF Checker does this comparison for you. It calculates the air the engine should be taking in, independently of the MAF, using the speed-density method, and compares it with the sensor’s reading:

• Measured air: the value the MAF sensor reports to the ECU (g/s).
• Calculated air: derived from engine speed, intake manifold pressure (MAP), intake air temperature (IAT), engine displacement and the volumetric efficiency (VE) of the engine type.
• Ratio = Measured ÷ Calculated. 0.90 – 1.10 is considered healthy.

The test runs at three engine speeds. How the deviation changes with RPM matters more than a single reading: a dirty MAF, a vacuum leak, a boost leak or a wrongly chosen engine type each leave a different signature. On petrol cars the app also cross-checks the result against fuel trim.

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What you need

• An ELM327 OBD-II adapter: Bluetooth (classic, SPP) or WiFi. A Bluetooth adapter must first be paired in the phone’s Bluetooth settings (PIN is usually 1234 or 0000).
• Android 6.0 or later on a phone or an in-car head unit.
• The vehicle must publish MAF data over standard OBD-II. On some brands MAF data is manufacturer-specific; generic OBD apps cannot read it even though the sensor is fitted. The app checks this for free.
• The calculation also needs RPM and MAP; IAT, coolant temperature, EGR, barometric pressure and fuel trim are used when available.
• A warm engine (coolant at least 70 °C), with the car parked, handbrake on and the gearbox in P or neutral.

First-time setup

1. Connection type: choose Bluetooth or WiFi under Settings → Connection. For WiFi adapters, first connect the phone to the adapter’s WiFi network; the address is usually 192.168.0.10 : 35000.
2. Engine type: under Settings → Engine type, choose the profile that best matches your engine (see the table below). The accuracy of the calculation depends heavily on this choice.
3. Engine modifications: tick EGR, DPF or catalytic/downpipe delete only if they have really been removed. With EGR delete ticked, the EGR reading is ignored; with DPF or catalyst delete, exhaust back-pressure is lower, so the calculation is corrected upwards by a few percent.
4. Engine size: enter the displacement in cc on the main screen (e.g. 1598, 2148, 2996). Use the value from the registration document or the engine label.
5. You usually don’t need to change the ELM327 timing settings. If an older K-line car returns no data, try increasing the response timeout.

Which engine type should I choose?

• Naturally aspirated, 2 valve/Cyl. — SOHC, older 8-valve engines (VE 0.80 / 0.82 / 0.83)
• Naturally aspirated, 4 valve/Cyl. — DOHC, 16-valve engines without variable cam timing (VE 0.76 / 0.82 / 0.86)
• Naturally aspirated + VVT — CVVT, VVT-i, VTEC, VANOS and similar variable-timing engines (e.g. Mercedes M272) (VE 0.72 / 0.70 / 0.70)
• Turbo / Supercharged — Turbocharged and supercharged petrol engines (VE 0.74 / 0.80 / 0.83)
• Turbo Diesel / Diesel — CRDi, TDI, CDI, dCi and other diesels (test speeds 1000 / 1800 / 2500) (VE 0.86 / 0.87 / 0.91)
• Custom / manual — If you know your engine’s VE you can enter a single value (VE your value)
• This vehicle (calibrated) — A reference saved from your own healthy car (see below) (VE specific to your car)

The VE values are for no-load test conditions (parked, steady RPM) and are typical figures. Engines can differ by a few percent; for the most accurate result, do a reference calibration.

Running the test

1. Bring the engine to operating temperature. Switch off consumers such as the A/C, headlights and heater.
2. Switch the ignition on and connect without starting the engine. The app then compares the MAP sensor with barometric pressure; with the engine off they should be equal. If they differ by more than 3 kPa, it warns that the MAP sensor is suspect, since the test relies on MAP data.
3. Start the engine. The live data card shows RPM, MAP, IAT, MAF, EGR and coolant temperature. If the car does not publish MAF data, the app tells you right away.
4. Check the engine size and press “Measure: 1000 RPM (step 1 of 3)”. If the coolant is below 70 °C the app warns you; you can wait or measure anyway.
5. Use the accelerator to bring the RPM to the target and hold it steady. Once the RPM has stayed near the target for 1.5 seconds, the app collects 8 samples and uses their median. A single outlier does not spoil the measurement; if the RPM leaves the band twice in a row, collection starts over.
6. When a step is done, the next target is shown: 2000 and 3000 rpm. For diesels the targets are 1800 and 2500 rpm, because many diesels limit RPM to about 2500 in P/N.
7. If the RPM cannot be stabilised, you get Retry and Skip this step. Measurements at two engine speeds are enough for the overall verdict.
8. After the three steps, each speed’s result and the overall verdict are shown. Press “Start new test” to measure again.

Accepted RPM band: ±100 around 1000, ±200 around 2000, ±250 around 3000 rpm.

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Reading the results

Each engine speed produces a result card:

• Header: the actual RPM of the measurement, the VE used at that RPM and the measurement quality. Reliable reading: the RPM stayed within the band while sampling. Unstable reading: the RPM fluctuated; the result is still used, but repeating it is a good idea.
• Measured / Calculated / Ratio: the air measured by the MAF, the air calculated by speed-density, and their ratio. The percentage in brackets is the deviation.
• Fuel trim (petrol cars): the sum of short- and long-term trim. On engines with two banks, the two banks are averaged.

• HEALTHY: Ratio between 0.90 and 1.10; measured and calculated agree.
• READING LOW: Ratio below 0.90. The MAF may be dirty or weak, or air may be leaking in after the MAF.
• READING HIGH: Ratio above 1.10. The MAF may be over-reading, or the MAP/IAT data is suspect.
• INVALID: EGR is more than 15% open. Exhaust gas displaces fresh air, so this measurement is not evaluated.

A single red line is not a fault on its own. Judge it together with the pattern across engine speeds and the fuel trim.

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Overall verdict: patterns

The app evaluates all engine speeds together and picks a pattern based on the size and direction of the deviation:

• Healthy: The deviation is within limits at every engine speed.
• Increasingly low with RPM: Measured falls further behind calculated as RPM rises. Dirty/weak MAF, a clogged air filter or a vacuum leak between the MAF and the engine. Check the filter and intake hoses first, then consider cleaning the MAF.
• Increasingly high with RPM: Measured exceeds calculated more and more as RPM rises. A boost-side leak on a turbo engine, a MAP/IAT fault or a MAF that over-reads at high flow.
• Similarly low at every speed: Check the engine size and vehicle type first. If they are correct, the MAF may be under-reading by a constant factor across the range.
• High at every speed: IAT read incorrectly (mirroring coolant), a faulty MAP sensor, a boost leak or a MAF that over-reads.
• Profile mismatch: The deviation is larger at low RPM or changes direction across the range. This is usually not a sensor fault but the chosen VE profile not quite fitting the engine. Review the vehicle type or do a reference calibration.
• Inconsistent / mixed: The results do not form a clear pattern. Warm the engine fully and measure again, holding each speed steady.

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Cross-check with fuel trim (petrol)

On petrol engines the ECU constantly corrects the mixture using the oxygen sensor. If the MAF really under-reads, the engine gets extra air, the mixture goes lean and the ECU adds fuel (positive trim). If the MAF over-reads, the ECU pulls fuel (negative trim). So fuel trim tells you whether the ratio comes from the sensor or from the VE estimate:

• Low ratio + trim +5% or more: Confirms the MAF is under-reading (dirty MAF or a leak after the MAF).
• Low ratio + neutral or negative trim: The ECU is not adding fuel; the sensor is most likely healthy and the deviation comes from the VE estimate.
• High ratio + trim −5% or less: Confirms the MAF is over-reading.
• High ratio + neutral or positive trim: The ECU is not pulling fuel; the deviation most likely comes from the VE estimate.
• Normal ratio + trim ±10% or more: The MAF looks healthy; the problem may lie elsewhere: fuel pressure, injectors, oxygen sensor or an exhaust leak.

This check is not done on diesel engines. On LPG-converted cars, run the test on petrol; the LPG tune affects fuel trim.

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Reference calibration (most accurate)

The built-in profiles are typical values. For a car-specific and much more accurate measurement, you can save a reference on a car whose MAF you know is healthy:

1. Complete the test on the healthy car.
2. Press “Save this vehicle as a healthy reference” below the results. The measurements are taken as a “healthy MAF” and VE values specific to this engine are saved.
3. The vehicle type switches to “This vehicle (calibrated)” automatically. Later tests use these values.

Only save if you are sure the MAF is healthy (new or original sensor, a car running without issues). If you calibrate with a faulty sensor, later tests will not detect the fault. The app warns you if fuel trim exceeds ±5%, and also if the engine size differs from the saved one. You can delete the calibration under Settings → Engine type.

Free features and the full test

• Free: connecting to the adapter, live data (RPM, MAP, IAT, MAF, EGR, coolant) and seeing whether your car publishes MAF data.
• Full test (one-time purchase): the three-step measurement, pattern verdict, fuel-trim cross-check and reference calibration. Buy once, use without limits.

Before buying, start the engine and check that a MAF value appears in live data. On a car that does not send MAF data, the app warns you before purchase.

Frequently asked questions

It says “This vehicle does not publish MAF data”.
The ECU does not share the MAF value over standard OBD-II. Even with the sensor fitted, the test cannot run in that case. On some brands this data can only be read with manufacturer-specific protocols.

MAF always shows 0 g/s with the engine running.
The car reports MAF support but does not send a valid value. The data is unusable, so the test cannot run.

I get a MAP warning at idle.
On a petrol engine, if MAP at idle is very close to atmospheric pressure, the car may not be publishing manifold pressure correctly over the standard PID. On a diesel, MAP higher than expected at idle may point to a turbo/leak issue or MAP drift. In both cases treat the results with caution.

My diesel won’t go up to 2500 rpm.
Some diesels limit RPM lower in P/N. Skip that step with Skip this step; the verdict uses the remaining speeds.

The result says “INVALID — EGR active”.
With EGR open during the measurement, exhaust gas displaces fresh air and the calculation breaks down. Once the engine is fully warm and at steady RPM, EGR usually closes; repeat the test. Tick EGR delete only if the EGR has really been removed.

On a V8 the ratio is about 0.50 at every speed.
Some V8s have a separate MAF for each bank, and the standard PID may report only one of them. This is not a sensor fault.

The result says “READING LOW”. Should I replace the MAF straight away?
No. Check fuel trim first, inspect the air filter and intake hoses, and if needed clean the MAF with a suitable spray and repeat the test. If fuel trim is neutral, the deviation most likely comes from the profile.

The adapter shows up but won’t connect.
Make sure the adapter is paired with the phone and the ignition is on, and update the app to the latest version. Close any other app connected to the adapter.

Important note

Web version of this guide: www.mscsi.net/mafcheck
Download: Google Play — MAF Checker
Feel free to post your questions, suggestions and bug reports in this thread.