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How to Diagnose & Rectify Airbag Problems With A Diagnostic Computer

 

1) Read the airbag system fault codes

2) Clear the codes

3) Re-read the codes and make a note of the results. These codes will be the remaining active fault codes which need further investigation.

 

Fault Code Results & Causes

 

Diagnostic Trouble Code:  ECU Defective

If you have one fault code only which relates to a defective ecu then it would appear there is something wrong with the airbag module. Sometimes they just won't accept re-programming and are deemed beyond repair. If this is the case then you should follow our returns procedure & return your module for a refund by clicking below.

Airbag Reset Returns Procedure

 

D.T.C :  ECU Defective + Others

If you have the fault code for a defective ecu and this is amongst other fault codes then it could be a spurious code. Some modules do display this code until the other active faults have been rectified. This is particularly common with Honda, Renault & some GM cars like Vauxhall / Opel. You should try to work through the other faults to see if you can eliminate them. If you'd like to test the airbag module you could disconnect a known working component like a seatbelt pre-tensioner / crash sensor or airbag and it should introduce a new fault code to the modules memory, if it doesn't and the other fault codes don't seem to correspond to the damage or repairs then you may suspect the airbag module at fault & return it for a refund, see returns procedure on the link above.

 

D.T.C : Low resistance on a circuit

A fault code for low resistance on a particular circuit would usually indicate a short circuit which could be caused by water ingress or a trapped or chaffed wire. You should check the wiring for that component. Faults usually occur where people have been working or where heat has been produced by airbag deployment.

We have also known low resistance fault codes to show up when airbag connectors have not been pushed fully home and locked into place by the security tab. You would expect this to cause high resistance but this does occur and should be investigated.

If the short is to earth you could disconnect the component and airbag module so the wiring is isolated and with a multi-meter see if the circuit in question is meeting earth, this would indicate the wire is trapped and is making contact with the vehicle body.

Airbag connectors also contain a ferrite suppressor to protect against accidental deployment. Sometimes these can become cracked and this can cause a low resistance, in this case the connector should be renewed. Sometimes the coil winding around the suppressor can burn out like a fuse and make the circuit go open circuit.

 

D.T.C : High resistance on a circuit

A fault code for high resistance on a particular circuit usually indicates an open circuit or bad connection. This could be being caused by a faulty or deployed component, a connector not connected properly, damaged or trapped wiring, water ingress or corrosion in a connector/ wire. The most common problem is connectors not fully home or burnt out. Most airbag connectors can be dismantled and contain a ferrite suppressor as mentioned above. If the coil winding around the suppressor burns out then the circuit will go open circuit and create high resistance.

 

D.T.C : No signal

No signal fault codes usually relate to crash or impact sensors. It basically means the airbag module can not see it and this could be caused by damaged wiring or more often because the crash sensor is of the type that needs replacing after airbag deployment. Please refer to the vehicle manufacturers data for specific information. Honda's usually require crash sensors replacing.

 

D.T.C : Short circuit

This usually means that the circuit in question has a short on it. Somewhere it is breached so this could be due to burnt wiring or trapped wires where something has been replaced or deployed, you should also check the connectors are connected properly.

 

D.T.C : Crash data stored

This means the module contains crash data and needs resetting or replacing.

 

D.T.C : Longitudinal acceleration threshold exceeded

This is crash data, the same as above.

 

D.T.C : Low capacitance

This usually means that the capacitance on a particular component is out of tolerance which can usually be put down to a faulty component or wiring. 

 

D.T.C : No Communication

This means the diagnostic computer cannot establish communication with the airbag module. This could be because of a problem with the diagnostic computer or it could be because the module is not powered up which could be being caused by a bad earth to the module or no power feed to the module. You should ensure the module is bolted down with any earth straps attached and it is receiving an earth. Also check that the appropriate wire at the module is receiving battery voltage and it is 12 - 14 volts d.c. If the module is receiving power and earth then suspect the module at fault, sometimes incorrect programming can cause communication failure. If you have no power to the module then you need to check fuses and power distribution. If you have no earth, then you need to check earth distribution.

 

Airbag Terminology

Airbag system - whole airbag system including components, wiring and airbag control module

Squib - usually relates to an airbag connector

Ferrite suppressor - suppressor inside the airbag connector 

Airbag Module or ECU - electronic airbag control unit which manages the airbag system, it continuously looks for faults and will illuminate the airbag light if it finds one. It is also responsible for deploying airbag components in the event of an accident.

Crash / impact sensor - small sensors located on the car which detect & then inform the airbag module of an impact to allow it to decide which components need to be deployed, if any.

Pre-tensioner - seatbelt pretensioner either fitted to the seatbelt reel or buckle

Inner pre-tensioner - seatbelt pre-tensioner on the buckles

Outer pre-tensioner - seatbelt pre-tensioner in the reels or on the inner sills

Lap belt pre-tensioner - seatbelt pre-tensioner on the inner sills, buckles or rear inner buckles

Thorax airbag - side impact airbag in the seat or door

Head airbag or curtain - airbag in the roof lining

Knee airbag - airbag under the steering column or glovebox

Drivers airbag - steering wheel airbag

Passenger airbag - dashboard airbag

Side airbag - seat or door airbag

L.H or R.H airbag - this can refer to left hand or right hand and can refer to the side airbag or frontal airbags depending on vehicle manufacturer so don't get caught out. L.H Airbag may mean dashboard airbag. Side airbags usually are referred to as 'side airbag with LH or RH pre-ceeding.

TIP - Be aware some diagnostic computers can mix up left hand and right hand, we have been presented with fault codes for a fault on say the left hand seatbelt tensioner which you would assume is the passenger side and in fact it was the drivers side at fault. We have found this on Peugeot and Honda using both launch and bosch diagnostics, so don't get caught out on that one. It's a good idea to disconnect the opposite side to see if the fault reading is getting the sides correct.

Airbag de-activation switch - switch or key switch to disable passenger airbag

Seat position sensor - small sensor located on the seat rail

Airbag MIL or tell tale - airbag warning lamp

Stage 1 - This refers to the stage 1 wiring or connector to an airbag. Some airbags have one or two connectors which are named stage 1 & stage 2. Airbags which are 2 stage means they have 2 different levels of deployment depending on the severity of the impact. Think of it like this, minor impact inflates stage 1 which partially inflates the airbag & for a major impact the airbag will use stage 1 & 2 for full inflation.

Stage 2 - This refers to the circuit or connector for the stage 2 circuit of an airbag. Airbag connectors will usually be marked with 1 & 2 or A & B respectively so you can identify which circuit is at fault. The marking is sometimes printed onto the airbag adjacent to the connector socket.

 

Common Faults

Airbag faults are usually very easy to rectify once you have a fault code and you know which circuit you need to be looking at.

Apply a little common sense, if you have a fault code for drivers airbag then remember it means that circuit and not just the airbag itself. The circuit consists of module - wiring - component so the fault could be any along that line.

Usually faults are to be found where people have been working or a component has been deployed. In most cases the airbag system would have been fine prior to the accident and the airbag system would be just as it left the factory, so don't go ripping out dashboards and lifting carpets to trace yards or wiring when it hasn't been touched.

Think about what has been triggered and where human hands have been and you won't be far off finding the fault.

Here are some common faults we find...

Blown Fuses - check airbag fuses

Bad earth - check earths to the airbag module

Airbag or seatbelt not connected properly - Check the component in question is connected properly, especially the connectors with security tabs. Many people push the connector into the airbag by pressing the back of the plug. What this does is it locks in the tab before the connector has gone fully home and in turn creates a bad connection. It may click and feel like it's in but so many times we come across this problem. To install correctly, release the security tab, hold the connector on the sides and push it fully home, then lastly click in the security tab to lock the connector in.

Burnt connector or wiring - due to extreme heat when the airbag or seatbelt was deployed, examine connectors internally by dismantling them.

Trapped wires - wiring trapped under seat rails, behind dashboards, behind airbags, against the seat frame, again think about what has been changed and where people have been working.

Clock spring open circuit -  check the clock spring , also known as spiral contact or drivers airbag squib behind the steering wheel, quite often they go open circuit after airbag deployment. To check it would need to disconnect the connectors at the airbag and under the steering column and check continuity through the unit with a multimeter.

Seat belt tensioner not replaced - if frontal airbag/s have gone then in most cases so will the seatbelt tensioners. If the car has had a rear impact then the belts may have deployed. Tensioners don't always go off with side impacts, this depends on the vehicle manufacturer and you should refer to them for this information. Seat belt tensioners can be on the seat belt reels, buckles or both, some cars have two tensioners on each front seat and rear tensioners also. Typically, the fault code for a blown pre-tensioner would show has high resistance / open circuit or as triggered.

Buckle type pre-tensioners tend to pull down towards the floor of the car so you may find they are not equal lengths, this is a visual indication they have deployed, also look for damaged trims around the seat base, quite often a deploying pre-tesnioner will break trims. Also, be aware sometimes buckle type pretensioners can deploy but look perfect.

Reel type pre-tensioners do sometimes lock up which is a clear indication that it has deployed, however sometimes the seatbelt reel will release itself so the seatbelt appears to be working and many people assume the pre-tensioner is ok. This is not always the case and a fault code would reveal a deployed pre-tensioner.

Crash sensor not replaced - some manufacturers require crash sensors are to be changed after deployment of airbag/s. A fault code would be present for this if it required renewing.

Airbag module connector - check you haven't bent the pins on the module when fitting the connector. Also check that the plastic locating plugs haven't broken away. These actually act to open a busbar inside the module connector when you plug it it in. If they are missing you will get a short circuit on that component which the circuit controls.

Bad connections - if you have a high resistance on a component circuit then disconnect and reconnect the connectors, for example the wiring under seats for seatbelt pre-tensioners are prone for corroding due to moisture in the carpets from wet feet. Condensation can cause corrosion inside connectors & this will create a increased resistance in that circuit.

 

From experience here are our top 3 most common faults:

1) Seat belt pre-tensioners not replaced

2) Connectors not fit properly

3) Damaged connectors

 

Testing Circuits & Safety

Firstly, here are some important safety tips:

Never probe airbag wiring with a power probe or multi-meter unless you have isolated the circuit by disconnecting both the airbag module and airbag component. Put a live feed into the wrong wire and bang ! you've made an expensive mistake !

Don't be afraid of working on airbag systems, as long as you apply common sense and work with the ignition off and key out when checking things then you won't have a problem.

Never leave airbag modules loose - some contain internal impact sensors so knocking the module could trigger airbags.

Never knock an airbag module or crash sensor with the ignition live

Never knock near a module or crash sensor with the ignition live 

Never leave airbags hanging loose

Always work side on to airbags and never directly facing them. Accidental deployment can cause serious injury.

Never try to short out or trick crash sensors - you may trigger an airbag

Never try to cheat by avoiding to replace airbags or seatbelt pre-tensioners

Never dismantle a seat belt pre-tensioners, seatbelts are very dangerous, the reels contain coiled springs which can easily spring out & cause serious injury to your face & eyes.

 

Ideally when you are working on an airbag system you should have the wiring diagram for that vehicle. Obviously this is not always possible to obtain so identifying wires to components may prove difficult.

Having said that, in most cases when dealing with a damage repaired car we find we rarely need to use a multi-meter to check the resistance in circuits or to check for short circuits.

Nearly always you can visually see where the fault lies and nearly always it is where somebody has been working and disturbed something.

Once you have the fault code and you know which circuit you are working on then you are not far off finding the fault.

Check wires for signs of melting or chaffing where they may have been trapped. Also remove trunking and insulation, sometimes wires are damaged inside the trunking and this hides the damage.

Check connectors for damage both externally and internally.

Check connectors are located home and security tabs are clicked in.

Disconnecting & re-connecting connectors can resolve an high resistance, in these cases you should apply contact cleaner to that connection or better still remove the multiplug and solder the wires together, ensuring you do them one by one and join the correct wires together, then of course insulate them individually.

The best piece of advice we can give you is apply common sense and look where people have been before you and think about what has been removed or replaced during the repair.

If you need further advice on testing circuits with a multi meter then we would advise researching on you tube as there are many tutorial videos for testing electronic circuits and components.

If you feel the fault finding process may be too involved, then find a good local garage capable of dealing with modern electronic systems.

We hope you resolve your airbag issues and find this website informative.

 

 

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Example Faults & Fixes

Ex 1: Vauxhall Astra

Airbag light on  -  read fault codes  - revealed code " B1234 passenger airbag stage 1 high resistance "

Removed glove box to gain access to passenger airbag, examined both connectors, identified stage 1 connector, check security & found connector loose, removed security tab, released connector, refit connector ensuring fully home then inserted security tab.

Re-read fault codes, erased fault codes, re-read fault codes  - NO DTC - exit airbags - airbag light out - fault fixed - Vauxhall Astra


Ex 2: Renault Megane

Airbag light on - read fault codes - revealed code  " B1234 drivers airbag open circuit "

Ignition off, key out, removed drivers airbag, checked connector to airbag - looks ok, unplugged drivers airbag, removed steering column trims and disconnected lower clock spring connector, with multi meter checked continuity from under column clock spring connector through to drivers airbag connector, found continuity in first wire ok, found open circuit on second wire - suspect clock spring damaged & open circuit.

Replaced clock spring & refit drivers airbag and lower connector - read fault codes - erase fault codes - NO DTC - exit airbags - airbag light out - fault fixed. 


Ex 3: Peugeot 307

Airbag light on - read fault codes - revealed code  " B1234 open circuit or increased resistance on passenger curtain or seat airbag"

Ign off key out, gained access to curtain airbag, checked security of connector and wiring, airbag looks like it has been replaced but connector is secure and wiring not trapped or damaged, dismantled connector - ferrite suppressor intact - tested coil with multi meter and we have continuity, rechecked fault codes with curtain disconnected - no change - reconnect curtain airbag and turn attention to seat, examine wiring under passenger seat immediately find wire trapped under seat rail - wish I'd had a quick look before gaining access to curtain but never mind - remove passenger seat - release & repair damaged wiring - refit seat - secure wiring correctly - read - erase and re-read fault codes - NO DTC - exit airbags - airbag light off - fault fixed. 

NOTE: On this car curtain and seat airbag shared same circuit hence fault code description mentioning both components.


Ex 4: Volvo V70

Airbag light on - read fault codes - revealed code " B1234 low capacitance on L.H rear seat belt pretensioner".

Ign off key out - dismantled L H rear seat to gain access to seatbelt reel & pre-tensioner mounted in seat frame. Examined wire and found connector has been cut off and wires bridged - somebody has been here before and tried to cheat !

Replaced connector & refit, read fault codes - erase & re-read - found 1 fault code still remaining but has changed to:

" B1234 high resistance on L.H rear seatbelt " - suspect LHR seat belt pre-tensioner deployed, seatbelt reel smells of gunpowder, extended seatbelt and found stretch marks on the seat belt also found seat belt rattles - definitely deployed !

Replaced LHR seatbelt reel - read - erase -read fault codes - NO DTC fault fixed airbag light off. 


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