MOT advisory explained
“Position lamp slightly affected by the operation of another lamp”
A tail light that dims, flickers or brightens when the indicator flashes beside it. It is the most recognisable electrical fault in the MOT, it is almost always a bad earth in the rear lamp cluster, and on one small French car it turns up twenty-four times more often than average.
Position lamps are the side lights, the small red lamps at the rear and white ones at the front that show where the car is. The tester switched them on, then operated the indicators, and one of the rear lamps changed as the indicator flashed. It still works, so it passes, but the two circuits are interfering with each other.
3.9% fail on a position lamp at the next test and only 12.7% are advised again. An earth fault gets worse as the connection corrodes further, and it usually takes the indicator or brake light with it before the tail light itself goes.
Nearly all of these are the rear lamps. “indicator”, “affected by indicator” and “with indicator” are three of the most common notes. If the indicator on that corner has also started flashing fast, the earth is already bad enough to be affecting it, and the next test will notice.
A “position lamp slightly affected by the operation of another lamp” advisory means a side or tail light visibly dims, flickers or brightens when another lamp, usually the indicator, is operated. The lamp still works, so the car passes. The cause is nearly always a poor earth in the rear lamp cluster. According to DVSA MOT data analysed by PriceMyRepair, 3.9% of 121,142 such advisories followed to the next test failed on a position lamp, and 12.7% were advised again.
Why one lamp affects another
A shared earth. All the bulbs in a rear lamp cluster return their current through one earth wire or one contact to the body. If that earth is corroded or loose, current from the indicator bulb looks for another way back, and finds it through the tail light filament. The tail light dims, brightens or flickers in time with the indicator. On some cars a brake light does the same when the indicator flashes.
Where it goes wrong. The earth contact is usually a spring blade or a screw at the back of the lamp unit, or a ring terminal bolted to the body inside the boot. Water gets in, the contact goes green, and the resistance climbs. Bulb holders with corroded contacts do the same thing one bulb at a time.
What the wordings cover. Five wordings are grouped here. The main one is the interference fault. “both” and “both sides” lead the notes because the earth serves both sides through the same loom on many cars. The others cover a position lamp with a coating on the lens or minor damage, recorded in the same section of the test.
Will it fail next year?
Five wordings, of which the first accounts for most, have been recorded 330,286 times. 121,142 can be followed to the same vehicle’s next MOT.
| What happened at the next test | Share |
|---|---|
| Failed on the same component | 3.9% |
| Same advisory recorded again | 12.7% |
According to DVSA MOT data analysed by PriceMyRepair: 121,142 advisories followed to the same vehicle’s next test. Rows can overlap.
For a lamp that is still working, one in twenty-five failing next time is high. The interference wording on its own fails 5.0%. A discoloured indicator fails 6.1% and a dead high level brake light 5.1%, and both of those are static faults. This one is a connection getting worse.
The repeat rate is one of the lowest of any lighting advisory. It is either cleaned up, or something in the same cluster fails within the year.
| Advisory wording | Fails on same part | Advised again | Followed |
|---|---|---|---|
| Direction indicator slightly discoloured | 6.1% | 19.6% | 1,901,107 |
| Stop lamp(s) non-obligatory stop lamp not working | 5.1% | 26.3% | 526,670 |
| Headlamp has a product on the lens | 5.0% | 22.9% | 1,248,099 |
| Supplementary restraint system warning lamp does not illuminate | 4.7% | 26.9% | 87,604 |
| Position lamp slightly affected by the operation of another lamp | 3.9% | 12.7% | 121,142 |
| Registration plate deteriorated | 3.7% | 40.1% | 4,283,862 |
| Brake pedal wearing smooth | 1.4% | 26.2% | 320,684 |
There are too few cases under five years old to report. From 5–6 years the rate rises from 0.2 per 1,000 tests to 2.5 at 20 years and over, and the share that fail next time rises from 1.7% to 5.0%. Corroded earths are an age fault.
| Age of car | Per 1,000 tests | Fails on same part | Followed |
|---|---|---|---|
| 5–6 years | 0.2 | 1.7% | 5,606 |
| 7–9 years | 0.4 | 2.5% | 13,568 |
| 10–12 years | 0.7 | 3.7% | 22,376 |
| 13–15 years | 1.4 | 4.5% | 29,705 |
| 16–19 years | 2.2 | 5.0% | 29,236 |
| 20 years and over | 2.5 | 4.2% | 11,622 |
With the side lights on, put the indicator on and watch the tail light. Then press on the back of the lamp unit, or wiggle its connector, and see whether the flicker changes. If it does, the fault is at the lamp. If a firm push on a ring terminal inside the boot stops it, that is the earth.
The repair is cleaning the contact back to bright metal and applying a little contact grease, or replacing a corroded bulb holder. It is a short job once found. Some cars have a known weak earth point behind the rear lamp, and the garage may go straight to it.
If cleaning the earth does not cure it, and every lamp on that side is affected, the fault may be in the loom where it passes through the tailgate or into the body. That is a longer job.
Exact wording on your certificate
The advisory is the slight version. The failure is a lamp that is affected to the point where it could be misread by other drivers.
| Line on the certificate | Result | Code |
|---|---|---|
| Position lamp slightly affected by the operation of another lamp | Advisory — pass | 4.2.3 (c) |
| Position lamp has a product on the lens or light source but not adversely affecting light intensity | Advisory — pass | 4.2.3 (b) (ii) |
| Position lamp slightly affected by the operation of another lamp | Advisory — pass | 4.2.3 (b) |
| Position lamp damaged but function not impaired | Advisory — pass | 4.2.1 (b) |
| Position lamp damaged but function not impaired | Advisory — pass | 4.2.1 (c) (ii) |
| Position lamp adversely affected by the operation of another lamp | Major — fail | 4.2.3 (c) |
The nearside rear and offside rear between them account for more than eight in ten. Front side lights share a cluster with a headlamp and a separate earth, and rarely show this. Rear clusters carry the tail, brake, indicator, fog and reverse bulbs on one earth, and that earth sits in the wettest part of the car.
| Position written by the tester | Share of all instances |
|---|---|
| Nearside rear | 43% |
| Offside rear | 39% |
| Rear | 10% |
| No position recorded | 3% |
| Offside front | 2% |
| Nearside front | 2% |
From 330,286 recorded instances. Testers can add their own note after the standard wording. The most common on this advisory: “both” (4,896), “both sides” (3,407), “indicator” (2,272), “both rear” (2,018), “affected by indicator” (1,568), “with indicator” (1,460), “by indicator” (617), “nearside and offside” (375).
What it costs
| Job | Price fitted | Notes |
|---|---|---|
| Earth clean-up or bulb holder, DIY | £10–£40 | Contact cleaner, grease, sometimes a new holder |
Finding the fault is the labour. Once found, the repair is minutes and a few pounds of materials. A replacement rear lamp unit, if the holder is part of it, is priced by model. A loom repair through a tailgate is a longer diagnostic job. Other prices are on the repair costs index.
Can I still drive with this advisory?
Have it looked at within a few weeks. A rear lamp cluster with a failing earth can lose its brake light or indicator without warning, and those are the ones that matter to the car behind.
Enter your reg to see every lighting advisory on your car’s record. A position lamp advisory followed by an indicator or brake light failure on the same corner is one earth, seen twice.
Which cars get this advisory most?
The all-vehicle rate is 0.6 per 1,000 tests. On the models below it is four to twenty-four times that.
| Model | Per 1,000 tests | Vs all cars | Tests |
|---|---|---|---|
| Peugeot 206 | 14 | 24.3× | 1,061,894 |
| Fiat Punto | 5 | 9.4× | 1,031,944 |
| Fiat Panda | 5 | 8.7× | 1,004,369 |
| Renault Clio | 5 | 8.7× | 4,086,721 |
| Renault Master | 5 | 8.4× | 760,734 |
| Vauxhall Movano | 4 | 6.8× | 544,309 |
| Renault Megane | 4 | 6.6× | 1,634,315 |
| Renault Kangoo | 4 | 6.4× | 700,467 |
| Nissan Micra | 3 | 5.3× | 2,941,349 |
| Citroen Dispatch | 2 | 4.1× | 556,497 |
Model families are grouped by the first word of the DVSA model name, so “Transit” covers every Transit variant. Only models with at least 60,000 tests are ranked.
One car is in a class of its own. The Peugeot 206 gets this advisory 24.3 times as often as the average vehicle, more than twice the rate of the next model. Its rear lamp earth is a well-known weak point, and any garage that has worked on one knows where to look.
The rest is small French and Italian cars and Renault vans: the Fiat Punto, Fiat Panda, Renault Clio, Renault Master, Renault Megane and Renault Kangoo. Designs that earth the rear lamps through a contact in the lamp unit rather than a bolted terminal on the body.
What to do next
- Confirm which lamp and which trigger. Side lights on, indicator on, watch. Then brake lights. The pattern tells you which circuit is leaking into which.
- Check the bulb holder first. Take the bulb out and look at the contacts. Green or white deposits are the fault half the time.
- Then the earth. Follow the lamp’s earth wire to where it meets the body. Clean, tighten, grease.
- Do both sides. They corrode at the same rate. The other side is next year’s advisory.
Position lamp affected by another lamp FAQ
What does position lamp slightly affected by the operation of another lamp mean?
A side or tail light visibly dims, flickers or brightens when another lamp, usually the indicator, is switched on. The lamp still works, so it is an advisory and the car passes.
What causes a tail light to flicker with the indicator?
A poor earth connection in the rear lamp cluster. All the bulbs in the cluster share one earth, and when it corrodes, current from the indicator finds its way back through the tail light filament.
Will it fail the next MOT?
According to DVSA MOT data analysed by PriceMyRepair, 3.9% of 121,142 such advisories failed on a position lamp at the next test. Only 12.7% were advised again, because a corroding earth tends to fail something in the cluster within the year.
How do you fix a bad earth on a rear light?
Clean the earth contact or terminal back to bright metal, apply contact grease and refit. Corroded bulb holders are replaced. Finding the fault takes longer than fixing it. Materials cost £10–£40; garage time is extra. These are estimates and final quotes come from garages.
Is it safe to drive?
Yes. The lamp works. Have it fixed soon, because a failing earth can take the brake light or indicator on that corner with it.
Which cars get this advisory most?
The Peugeot 206, at 24.3 times the average rate, far ahead of anything else. The Fiat Punto, Fiat Panda and Renault Clio follow at around nine times.
How these figures were calculated. According to DVSA MOT data analysed by PriceMyRepair, from the DVSA anonymised MOT test results (Open Government Licence v3.0): 350,943,111 tests. This advisory was recorded 330,286 times; 121,142 of those were followed to the same vehicle’s next test between 180 and 730 days later. “Fails on the same part” means a failure at that next test under the same defect code and naming the same component, on either side of the vehicle. Testers do not always name a component the same way twice, so the figure is a floor. Method and how to cite.