Loom of doom: throttle cut-outs, limp mode and the connectors to check

What "loom of doom" means

Owners use "loom of doom" for the engine loom connectors, above all the pair of blue and black plugs behind the coolant expansion tank, whose contacts corrode or loosen and cause throttle cut-outs, limp mode, warning lights and odd running. Some owners also use it for the loom as a whole. It is the first thing most owners check, and often the cure.

What it looks like

• Throttle suddenly stops responding, usually with the engine warning light (EML). Switching off and restarting sometimes brings it back.
• Sometimes it is worse: the engine just idles and the throttle does nothing, even with no limp mode.
• Turbo: one owner described limp mode as the pedal feeling like it only gave about 10% and the turbo barely charging, usually just after a gear change.
• One owner lost the throttle regularly (about once every 15 miles) and had to clear the code each time, even after restarting.
• Owners noticed it after work that may have disturbed the loom (a new front clam, for example) and over bumps. One suspected cold or wet weather, which is only his guess.

Codes owners saw

• P1271: accelerator pedal position sensor 1-2 correlation. One owner's throttle body proved to be the cause of this code, and a new one cured it.
• One owner's full list: P1221 (throttle position sensor 1-2 circuit performance), P1275 (accelerator pedal sensor 1 circuit), P1271 and P1635 (5 V reference #1 circuit). Another owner said this combination "suggests a wiring fault".
• A Turbo owner whose first code reader showed nothing later found P1122 and P1120, both to do with the pedal, when another member read the car.
• For P1512 and a car that won't start, see P1512 fault code. For a code lookup see Reading fault codes.

Where to look first

Owners agree it is usually a connection rather than a failed part. One owner's order of likelihood (2011): 1) the blue and black plugs behind the coolant tank, 2) the ECU plugs, 3) the throttle body plug, 4) the throttle pedal connection. Another says the loom behind the expansion tank causes about 90% of these cases, and that cleaning those plugs could be part of the annual service. These are owner opinions, not statistics.

1. The two plugs behind the expansion (header) tank. Jack the car up, remove the nearside rear wheel and the arch liner. The two connectors (blue and black) are on the header tank bracket, though on one car they were on the gearbox. Disconnect them, clean them with contact cleaner, check the contacts and push them fully home. They are tight and hard to latch, so check they have clicked fully into place.
2. Look inside the plugs. One owner says the female terminals open up from the leverage of the wiring. Pull off the yellow grid piece on the female side and examine each terminal. If one or two have opened up, close them with a safety pin.
3. The throttle pedal connector. One owner found the problem in the connector just above the pedal. Another says unclip, clean and re-seat it.
4. The throttle body connector. Clean and check it the same way.
5. The ECU plugs. The worst case, according to one owner, is corroded pins on the ECU, which needs a new engine loom and ECU. Another owner found water in the waterproof connector between loom and ECU had corroded quite a few pins away.
6. The coil pack plug. One owner found that unplugging and firmly re-plugging it cleared the EML more than once. He couldn't explain why, and nobody else in the thread linked it to the throttle, so it may be a coincidence. He tightened its pins carefully. Don't overtighten spark plugs if you take them out: excess force can crack the head (one owner's warning).

Getting the ECU plugs off (NA)

One owner's description: a stock NA has a bar across the bottom of the ECU that stops the plugs coming off. Remove the two tamper-proof Torx bolts to take the bar off. Then press the plastic latch on top of the plug while lifting the silver arm. A black plastic slider moves along the bottom of the plug as you lift (this is what the bar blocks). Once the arm is at the end of its travel you may need to pull the slider the last bit by hand, and then the plug lifts off.

Cleaning and resetting

• Cleaner. Use contact cleaner (one owner uses a can of automotive contact cleaner and a fibreglass pen). When one owner asked about WD40, another said use contact spray instead.
• Clearing the light. You can pull fuse 8 for a while (owners say anywhere from 10 or 20 minutes up to half an hour or 40 minutes) to clear the code. One owner found the immobiliser lit on refitting the fuse, but cleared with the fob. A code reader clears the light faster.
• The light staying on after a fix, with the throttle working, may just be a retained code. One owner who cleaned the plugs saw the light stay on until he cleared it.
• A code reader was recommended by several: a cheap plug-in reader (about £20 in 2011) or a self-contained ScanGauge (about £100). The OBD port is in the footwell, slightly to the passenger side of centre, and is awkward to reach. One owner recommends a particular type of cheap reader (the U480) and says other versions don't work with this car. That is unchecked. A laptop cable (OP-COM) gives extra Vauxhall functions, according to another owner.

If cleaning doesn't cure it

• The pedal assembly. One garage swapped the pedal box and cleaned the connectors, and the car worked, though the fault came back briefly later and cleared with a reset. An owner who was told the next step was swapping the pedal assembly put the price at about £130. One owner thought the pedal potentiometer can't be replaced on its own. Another offered a near-new pedal assembly on loan.
• The throttle body. If the pedal is working, the throttle body may be failing to operate (Turbo owner's comment). One owner says a new throttle body cured P1271 on his. To see if the throttle body is moving, with the ignition on, take off the intake pipe and watch it open (another owner's suggestion).
• Wheel arch connectors cured repeated MAF replacements. One Turbo owner who had replaced the MAF every year, and had a garage suggest a new pedal sensor, cleaned the contacts behind the rear wheel arch and got the car back for the summer. See MAF sensor faults.
• Garages. Several owners say dealers and some garages tend to throw parts at it. One owner advised against paying for fault-finding on a problem that has stopped, and another recommended a diagnostics specialist rather than a dealer. If you bought the car from a dealer, one owner got the repair done under warranty (it took five weeks).
• Coil pack. If you replace it, one owner says to avoid cheap eBay packs and get a GM or Denso one.

NA rough running at the same time

An NA owner (2006) with a rough idle and one case of no throttle response was told to unclip, clean and re-seat the connectors at the pedal, the nearside rear arch by the expansion tank, and the throttle body, then pull fuse 8 for 10 to 20 minutes. Another owner pointed out that the throttle pedal sensor isn't in play at idle, so rough running at idle may be something else, such as the coil pack, fuelling or the MAP sensor. See MAP and lambda sensor faults.

What the two loom of doom plugs carry

One owner posted this list in 2019, taken from the wiring diagrams. It is a single source, so check it against your car before you rely on it. It is useful for tracing a no-start or a lambda fault to the plug. Early cars (before 2002) are said to have the post-cat lambda wiring in these plugs.

Getting at the plugs, and a longer-running case (2004 to 2007)

• Without taking the wheel off. One owner reached both plugs by unclipping the expansion tank (one clip, leave it connected and move it aside), squeezing the two clips that hold the connectors to the back of the bracket, and undoing the bracket's two 14 mm bolts. Lower the bracket into the engine bay and both plugs are there. On refitting, clip the connectors back onto the bracket before bolting it up. Another owner reached them through the rear wheel arch with the wheel and liner off.
• The pedal loom is in two halves. The cable from the pedal to the ECU is split at the expansion tank. A connector on the back of the bracket joins the two halves, and it takes quite some force to click back together.
• What P1271 means. The pedal sensor contains two potentiometers. If the ECU sees them disagree by too much, it drops the engine to tickover for safety. One owner's readings in the fault were 1.0 V on one sensor and 4.5 V on the other.
• Scan tools disagree. One owner's generic scan tool showed three unrelated codes (a timing reference signal, catalyst warm-up efficiency, and a gearbox solenoid code that makes no sense on this car) while the Tech2 tool showed P1271, and another piece of software mislabelled the same code. Treat the code text from cheap tools with care.
• Parts replacements that didn't cure it. That owner had the pedal sensor, throttle valve and ECU all replaced, and the fault came back. Cleaning the connectors at the pedal, the ECU and the split behind the tank helped for a while. A repair specialist said replacing all the connectors, both ends, had cured the cars they had seen. In a Turbo, one owner traced the same symptoms to water pooling round the ignition coil pack and cured it with the engine cover water shield and a new pack.
• The pedal plug. It has a locking slide that is stiff. One owner removed rubber boots over the plug first, then pushed the centre grey tooth of the plastic clip and slid it out (it is a stiff clip). On the NA ECU, an aluminium bar blocks the connector slide until you remove its two bolts (one is a Torx).

Revs stick, then jump (2018 thread)

A supercharged car idled fine cold, but after 3 or 4 minutes the revs rose normally to about 3,000, then did nothing as the pedal was pushed further, then jumped to 4,000 or 5,000 and wandered with the pedal held still. Lifting off at 4,000 stalled it, and it restarted and idled normally. The owner sprayed the plugs by the header tank several times.

• Advice from owners: make a log with the ignition on and press the pedal fully in a few times, then compare the two pedal sensor voltages and the throttle position. A log tells you what is wrong without spending money.
• The likely causes given were, in order: the black plug (the loom of doom), the ECU pins, the throttle body, the pedal. One owner had cleaned the blue plug above the passenger rear wheel with contact cleaner that has a moisture-repelling lubricant, and said it cured an erratic throttle.
• An owner with a similar fault replaced the pedal, did the loom connectors, and found the throttle body was the cause. Swapping in a borrowed throttle body is a quick test.
• Outcome. The owner replaced the coil pack, plugs, pedal and throttle body, cleaned all the plugs, and found the fault was the pre-cat lambda sensor. A bad lambda sensor can behave like a throttle fault, so do not replace the pedal and throttle body first (see MAP and lambda sensor faults). It is one case.
• The owner could not save a log because his software reported a corrupt programme, and unplugging the ECU showed a little moisture on the pins, but they were clean.

Throttle cutting out in the rain (2017 thread)

An owner found that after heavy rain the throttle cut out whenever he drove down the road. He had already cleaned the throttle body connector with contact cleaner and tried wiggling the ECU plugs. Cleaning the loom of doom connectors cured it. Others in the thread suggested:

• The coil pack and plugs. Two owners had misfires in heavy rain that went away with a new coil pack and plugs, and one said to take the coil pack off and look for water in the plug holes. His driver's side plug hole collects water in significant rain. The owner here had no misfire, so he left the coil alone.
• The primary lambda sensor connector. One owner whose connector got wet had the same symptoms.
• The blue plug won't come apart. The first reply was "press down the tab, pull apart". A second owner could part the black plug but not the blue one. Others said theirs are stiff but manageable, and to push a slim screwdriver down the sides and wiggle it. Nothing better was posted.

An engine light and a cutting-out engine (2003 thread)

A Speedster owner reported the engine management light came on at speed and the engine cut out at idle after a long, hot drive, and restarted straight away with the light still lit. It appeared every day on a holiday trip. It cleared for a couple of minutes after stopping at one point. Disconnecting the battery for 20 minutes, suggested by another owner, did not help. The thread does not give a cause. The car is drive-by-wire, not cable.