What to do if your injector light comes on - Alfa Romeo Forum
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What to do if your injector light comes on

Having just experienced this myself, the trawl through relevant threads on here highlighted that a sticky would be useful to bring together some advice into one place, and hopefully save people time and potentially money. People with more knowledge and understanding than me could add to this thread to make it an accurate resource.

The injector light can come on for a variety of reasons (not just injection related) and in the ideal situation you hook the car into Alfa’s diagnostic software to find out what/where the fault is. This isn’t always possible (eg. stuck at the side of the road) and can be misleading. There are a few things you can do yourself to locate the fault and save unnecessary costs and/or replacements.

The injector light can be associated with a change in engine performance. This can vary but usually falls into the following categories.

1. Flat spot in the mid-range, usually around 4000rpm
2. A 2-3 second delay between pressing the accelerator pedal and a rise in revs, which is slow and low powered (sometimes referred to as “limp-home mode”)
3. Loss of all power with revs refusing to rise above approx 1500rpm
4. No discernable change in performance
5. Problems only when engine is hot

The first thing to do is turn the engine off and restart – if this works, still consider a diagnostics test to prevent it happening again (fault codes are stored). Also read on regarding potentiometer cable connectors.

Number 1 is usually due to MAF failure, although this can cause a variety of different effects. Therefore it is sensible to exclude this whenever you see the injector light - simply unplug the MAF and restart the engine and if things improve then it is the cause. Note that the engine will not run quite to full power without the MAF connected. It is located by following the back air box out of the right hand side of the engine, which turns down vertically towards the air intake: the MAF is a black cylinder just before the intake with a plug attaching at about 1 o’clock. If the MAF is the cause, make sure it is replaced with the correct Bosch part for your model + year. Another cause of rough idling and flat spots is failed lambda sensors – again a diagnostics test should point towards this.

If disconnecting the MAF has no effect, it may be worth disconnecting the battery for 45 mins (negative lead), particularly if the light is on but performance is unaffected. Some report this can occasionally cure it, although again, a full diagnostics test should be undertaken soon afterwards. Others have tried full ECU resets (search under “ECU re-learn” for the thread on how to do this). Many report that leaving the car 24hrs clears the problem but this points towards a dodgy connection so could happen again.

Numbers 2 & 3 could be due to a sensor failure (eg crank, particularly no. 5), throttle potentiometer failure, or (more likely) a dodgy throttle potentiometer cable connection. The latter is suggested if things miraculously cure themselves intermittently, or if the problem occurs following a large bump/pothole. It may be fine for months at a time. The diagnostics will show a throttle potentiometer failure (on fly-by-wire cars, the earlier models were cable-throttles) but this is actually apparently rare. A non-Alfa specialist will then replace the unit and charge you well over 500 quid. More commonly, the connectors fail. The one that most people report as an easy fix is located in the passenger footwell just behind/adjacent to the centre console. Simply undo the 2 screws holding the right hand carpet to the side of the centre console and gently pull out the wires. Among others, there will be 2 with large plastic connectors in there (one of mine was buried deep behind the stereo), one white and one green. The white one is the throttle potentiometer cable: disconnect, clean up and spray with WD40 or contact cleaner. I positioned mine when replacing the carpet trim so that it’s within east reach for future problems. A few people have reported that fiddling with the connector close to the actual potentiometer can cure it: drop down the fuse box cover on the right of the driver’s footwell. The unit is a black cylinder like a big battery. Apparently nearby there’s a similar white connector. This can easily be done at the side of the road with just a Phillips screwdriver so can save time and a tow home. Many people now remove the connectors and hard-wire them instead.

Hope this helps, please add and correct bits then it can be put in the “How to” thread.


Last edited by jim3; 29-12-09 at 16:24.
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Good post Jim.

Can I add that a piece of freeware called Unidiag, and a generic OBD2/usb cable combined with a laptop will tell you exactly where the problem is by reading the fault codes. It will also allow you to reset them.
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I'm experiencing number 2 after replacing my sump again. I will check the poteomoter tomorrow and the (crank) sensor tomorrow

Many thanks, Kenny.
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