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Fault code

P0300 fault code.

This code comes up with 5 known faults.

Generic code

Code type

Engine and drivetrain

System

Misfire on several cylinders

The engine control unit detects a misfire from the momentary slowdown of the crankshaft. Here it found one on several cylinders, or could not tie it to a single cylinder. The car shakes, and the warning light may also flash. With a flashing light do not drive on, because the catalytic converter can be destroyed.

The code tells you what the control unit measured, not which part has failed. The same code has several possible causes.

Which known faults it comes up with

expensive4

M260 / M264 petrol: worn valve seat / valve guide, full cylinder head replacement

expensive

On the transverse M260 and longitudinal M264 four cylinder petrol engines the valve seat or the valve guide wears early. The symptom: check engine light, rough running, misfire on one or more cylinders. Sometimes without any fault code being stored.

Show the rest

According to the maker's official service bulletin (LI01.30-P-072328) the valve seat leaks. After the leak test, if a leak is found at the valves, the whole cylinder head has to be replaced: the valve seat cannot be refurbished. The roller width of the intake and exhaust side followers differs, and swapping them is prohibited.

In the United States the maker announced a 15 year / 150,000 mile (about 240,000 km) extended emissions warranty for the M260 group (MY2019 to 2022 A / CLA / GLA / GLB). Outside the United States that programme does not apply automatically, but the fault is the same mechanism. The GLB 250, the CLA 250 and the A 220 use this engine.

between 40,000 and 150,000 km

What to check

  • Ask for a fault code read: misfire codes of the P0300 / P030x kind and mixture formation codes are suspect
  • On a cold start watch for rough running and the check engine light
  • Ask whether the cylinder head has already been replaced: according to the service bulletin that is the repair, not valve lapping
2 more steps in the write-up

1.0 TSI: an injector that will not seal, fuel leaking into the cylinder

expensive

The injector does not close fully, and with the engine standing fuel drips into the cylinder. The symptoms are hard, long cranking (mostly after standing for a while), smoky first minutes, a hesitant idle, and dilution of the engine oil: the oil level goes higher, it smells of petrol, and the diluted oil lubricates worse. It shows up on cars built between 2018 and 2020.

What to check

  • Start it cold after standing for a while: several seconds of cranking and blue-white smoke are suspect
  • Pull the dipstick: a level rising above MAX and a petrol smell indicate dilution
  • Smell the oil filler: a petrol smell means fuel has got into the oil
2 more steps in the write-up

FSI direct injection: intake valve coking

expensive

With direct injection the fuel does not wash the intake valves: the petrol goes into the cylinder, not behind the valve. The oil mist and soot arriving through the EGR and the crankcase ventilation therefore build up on the valve stem and head, and from 80,000 to 100,000 km they restrict the flow. Symptoms are hesitation, loss of power, a rough cold start and misfire. It is not a part failure: it is a consequence of the design, and therefore it comes back. Cleaning (walnut or dry ice blasting) helps for a few tens of thousands of kilometres.

What to check

  • After a cold start watch the idle: rough running and shaking point to coked valves
  • Under load at low revs watch for hesitation
  • With a code reader look for per cylinder misfire (P0300 to P0304)
1 more steps in the write-up

Open deck block: the bore moves and the gasket cuts into itself

expensive

The block of the engine is of open deck design: the cylinder walls are not braced at the top. Under boost and heat the unbraced cylinder wall moves sideways, and this micro-movement puts a shearing load on the fire ring of the head gasket. The ring cuts further into itself with each thermal cycle, and because there is no solid backing underneath it between the cylinders, it extrudes into the coolant slots. In the end it lets pressurised gas and coolant through.

Show the rest

It is not the mileage that decides but the year of casting and the use. On the blocks cast between 2015 and 2019 there are grooves forming slots between the cylinders, and the risk is highest on those. In 2020 the maker revised the casting: in place of the open slots came small coolant passages drilled diagonally from the outer wall, and with that the metal backing returned. The number of failures dropped noticeably as a result.

The load is what counts, not the distance: the critical case is a heat-soaked restart after hard use.

The symptom is coolant going down with no visible dripping, white smoke, an uneven, stumbling idle, misfires (codes P0300 and P0316), mayonnaise-like oil and running hot.

The repair starts with a pressure test, then a revised gasket, a new cylinder head or a complete block replacement follows, depending on the extent of the damage. An engine that had already overheated before the repair can remain distorted, and then the coolant loss comes back.

What to check

  • Check the casting year of the block: a casting from between 2015 and 2019 is the risky one, post-2020 is the good one
  • If it is from between 2015 and 2019, ask for the repair history of the head gasket or proof of a block replacement
  • Look at the coolant level and ask whether it has had to be topped up
3 more steps in the write-up

comfort1

B5: early failure of the ignition coils

comfort

The per cylinder ignition coils fail early: jerking, misfire, a flashing engine warning light, limp mode. Typically several age at once, and after a replacement the new ones can go early too. The phenomenon occurs on every B5 engine, not only the turbocharged ones.

What to check

  • After a cold start watch whether the idle is even and whether there is shaking
  • Under load (overtaking, a slope) check whether it jerks
  • With a code reader look for per cylinder misfire (P0300 to P0306)
1 more steps in the write-up

Information, not an expert opinion

We write the descriptions, and we put the source next to them. A person reviews them before they go out. They are still information, not a technical expert opinion.

  • A fault on the list does not mean the car you are looking at has it.
  • A fault missing from the list does not mean the model is fault-free. Write-ups are made model by model. Not all of them are done.
  • You decide whether to buy. This is no substitute for an inspection. That needs a workshop and a history check.
Details in the terms