Fault codes after an LPG conversion: what they mean

Fitted LPG and the light came on. Usually the engine is not broken — the two systems are not agreeing. Which codes are expected and which are not.

The car ran without a single fault, an LPG system was fitted, and a week later the check light came on. The story is common enough to deserve its own piece.

Why it happens at all

The factory engine unit knows nothing about gas. It goes on working out how much petrol to inject, watching the oxygen sensors and drawing conclusions. Meanwhile the LPG unit intercepts the injector signals and doses gas by its own reckoning.

Two systems run one engine without talking to each other. While the calibration is exact, the factory unit notices nothing. As soon as there is slightly more or less gas than needed, it sees a mismatch — and sets a code.

Which codes come most often

Mixture. P0171 and P0174 for lean, P0172 for rich. The commonest group: the LPG unit is calibrated imprecisely and the oxygen sensor says so honestly. Cured by adjustment, not by parts.

Mixture by condition. P2177 and P2178 — the mismatch appears only under load while idle is fine (or the other way round, P2188). That is a valuable hint: the calibration is off in exactly the condition named.

Misfires. P0300 and the per-cylinder codes. Gas burns differently from petrol and needs a stronger spark. Plugs that would have lasted another year on petrol start missing on gas — there is a separate piece about plugs.

Catalyst. P0420. It appears later than the others and usually means the engine ran on a wrong mixture for a long time. A consequence, not a cause.

What is normal and what is not

Normal: the unit shows mixture codes once or twice in the first days while the LPG system is being tuned. They are cleared, you drive, and you see whether they return.

Not normal: the codes come back again and again after several tunings. Then it is not the calibration — look for an air leak, worn injectors or a faulty reducer.

Alarming: the light is FLASHING. Misfires on gas melt the catalyst just as on petrol, and faster — exhaust temperatures on gas are higher.

How to look for yourself

The main instrument here is fuel trims. Connect and look at them on petrol and on gas in turn:

• Trims near zero on petrol and far off on gas — the LPG calibration is at fault.
• Equally far off on both fuels — the trouble is in the engine itself and gas has nothing to do with it. Look for an air leak.

Making that distinction before visiting the installer tells you straight away whom to go to.

What not to do

• Unplug the oxygen sensor. People sometimes «cure» mixture codes that way. The unit then loses its feedback and doses fuel blind — consumption rises and the catalyst dies.
• Clear the code and drive on. Clearing fixes nothing, and incomplete monitors will block an emissions test.
• Replace parts by the code. A code names a place, not a part. On gas that is especially galling: usually nothing needs replacing, something needs adjusting.

In short

• The factory unit knows nothing about gas; the codes are two systems arguing.
• The commonest group is mixture codes; cured by tuning, not by parts.
• Plugs wear faster on gas, hence the misfires.
• Compare trims on petrol and on gas — that names the culprit at once.
• A flashing light on gas is worse: hotter exhaust kills the catalyst sooner.