The largest and best supported Jaguar cars enthusiast site on the Internet!

Serving Enthusiasts since 1993
The Jag-lovers Web

Currently with 3,166 members

Jag-lovers' Ebooks
- Jag-Lovers' xj40 Ebook -

12.12 - Fuel Fail 12 : Mass Air Flow sensor out of range ( Brett Gadzinsky,  )

Output below .2 volts or above 4.5 volts engine rpm 400 to 3000 rpm for 5 crankshaft revolutions.

If FF 12 is flagged, ecu uses throttle position sensor input for engine load indications instead of the maf sensor.

Possible faults :

- Old style unsealed relays for EMS main 1993 year cars and up.
- Open or short in wiring between maf and main ecu.
- High resistance connection between maf and ecu (dirty connector at maf sensor).
- Open circuit in wiring from main relay power (12 volts) to maf.
- Bad maf sensor.

The maf sensor is in the air intake tract, after the air filter and before the throttle. It uses air flow through a side port to cool and change the resistance of sensors, corrected for ambient air temps to measure the air flow through the intake tract. Since it has no moving parts, its reliable, but suffers from having its connector in a bad spot. Its also subject to contamination of the sensors by any type of liquid.

Inside the sensor is a circuit board with voltage regulators, and a lot of parts and IC chips. On the 1988 to 1992 year cars, the base idle mixture trim is built in, but not part of the actual maf circuitry. Sensors to 1991 had a separate ground wire to a stud on the maf.

To test the maf, warm the car up to operating temps. At idle in park (about 740rpm) the output signal should read 1.2 to 1.5 volts, the higher the reading, the higher the indicated air flow. If your maf sensor puts out about 1.2 volts at idle, it is indicating a low air flow, so will adjust the injector pulse time down
to keep the mixture in range. 1.2 volts is on the low side of a normal reading and may cause problems unless you are at a high altitude.

If you have a vacuum leak, the INDICATED air flow into the engine will be low, the ecu will reduce the injector pulse, and you will have a lean mixture and a poor idle.

To test the high air flow, measure the output voltage with the car in drive, full throttle, foot on brake. Only do this test for a very short time, and use great care the car does not get away from you. At full throttle the maf should read about 3.0 to 3.3 volts. This is the torque converter stall reading, actual full throttle high rpm will indicate higher.

The maf sensor has a lot of input on the mixture and indicated load range the engine is operating at. A small change in the signal denotes a large change in mixture.

Under normal operating conditions, the maf sensor and the coolant temp sensor set the mixture and the way the engine runs.


sepbar.jpg (3094 bytes)

This section currently maintained by Pascal Gademer; questions, comments, submissions and suggestions, email


Please help support the move to the new site, and DONATE what you can.
A big Thank You to those who have donated already!



Go to our Homepage
Improve your Jag-lovers experience with the Mozilla FireFox Browser!

  View the latest posts from our Forums via an RSS Feed!

Jag-loversTM Ltd / JagWEBTM 1993 - 2022
All rights reserved. Jag-lovers is supported by JagWEBTM
For Terms of Use and General Rules see our Disclaimer
Use of the Jag-lovers logo or trademark name on sites other than Jag-lovers itself in a manner implying endorsement of commercial activities whatsoever is prohibited. Sections of this Web Site may publish members and visitors comments, opinion and photographs/images - Jag-lovers Ltd does not assume or have any responsibility or any liability for members comments or opinions, nor does it claim ownership or copyright of any material that belongs to the original poster including images. The word 'Jaguar' and the leaping cat device, whether used separately or in combination, are registered trademarks and are the property of Jaguar Cars, England. Some images may also be Jaguar Cars. Mirroring or downloading of this site or the publication of material or any extracts therefrom in original or altered form from these pages onto other sites (including reproduction by any other Jaguar enthusiast sites) without express permission violates Jag-lovers Ltd copyright and is prohibited
Go to our Homepage
Your Browser is: CCBot/2.0 (, IP Address logged as on 27th Jun 2022 06:23:30