If you are trying to figure out why your car runs poorly on gas, our symptom-first troubleshooting guide walks you through it step by step. This page takes a different approach. We go through every major component in the LPG system, explain what typically fails on it and how the failure announces itself. Think of it as a component encyclopedia from our workshop floor.
Liquid and Vapour Phase Filters
Filters are the most common and cheapest failure to prevent, yet they remain one of the top reasons vehicles come in with lost power on gas. Every LPG system has at least two filters, one for the liquid phase (before the reducer) and one for the vapour phase (after it). Fuel quality at filling stations varies, and the contaminants the filter catches gradually reduce flow.
A clogged liquid-phase filter starves the reducer, so the engine loses power under load, especially on the motorway or when overtaking. A clogged vapour-phase filter causes similar symptoms but also adds rough idling. The tricky part is that degradation happens gradually; the driver adapts and does not notice until the car starts jerking or stalling. Replacing filters is a fifteen-minute job that should happen every 10,000 to 15,000 km depending on local fuel quality. For more on timing, see our service interval guide.
Reducer (Vaporiser)
The reducer is the heart of the LPG system. It converts liquid gas into vapour using heat from the engine coolant and regulates the pressure of the gas heading toward the injectors. Inside the reducer sit membranes (usually special rubber or silicone) and springs that control pressure.
The membrane ages, loses elasticity and eventually cracks or warps. When that happens the reducer cannot hold stable pressure, the engine gets the wrong amount of gas, and you feel jerking, power loss or a reversion to petrol. Springs also weaken over time, pushing the working pressure outside factory values. Another frequent issue is the coolant connection, a leak or blockage in the hose that brings warm coolant from the engine cooling circuit. Without enough heat the reducer cannot vaporise the fuel properly, and that creates the well-known "car runs badly on gas until it warms up" problem. In winter these symptoms are far more pronounced because the starting temperature is lower and the reducer needs longer to reach operating temperature.
Reducer servicing means replacing membranes and gaskets and checking springs, typically every 80,000 to 120,000 km or sooner if symptoms appear.
LPG Injectors
Injectors are precision electromagnetic valves that meter gas into the intake manifold. They go through millions of open-close cycles, and over time seals wear, springs weaken and deposits from the fuel can stick moving parts together.
The most common symptom of worn injectors is jerking while driving, especially at constant motorway speed. It happens because one or two injectors dose differently from the rest, so cylinders receive unequal amounts of gas. The engine runs unevenly, which the driver feels as a light jerk or shudder. An injector that sticks responds more slowly to signals from the LPG ECU, creating a dosing lag. In advanced wear the gas consumption rises noticeably because the system compensates for poor atomisation with a larger volume of fuel.
Ultrasonic cleaning usually restores performance if wear is in the early stages. With more serious degradation, complete set replacement is needed because mixing old and new injectors creates a dosing imbalance between cylinders.
Multivalve and Tank
The multivalve is the assembly mounted on the LPG tank that combines the fill valve, the gas take-off valve, a safety valve and the level sensor. The most common multivalve problem is a level sensor that lies. The gauge on the dashboard shows half a tank, then gas runs out after twenty kilometres, or conversely it always reads full. The sensor works on a float or magnetic principle and is subject to wear.
Another issue is a valve that does not seat fully, which can cause the tank not to fill completely or gas to seep slowly (recognisable by the mercaptan smell in the boot). It is important to understand that the LPG tank has a fill limit of 80 percent of capacity. That is not a system fault but a safety measure that leaves room for gas expansion as temperature changes. If the gauge never reads "full," that may be entirely normal.
The tank itself rarely causes problems within its certification period, but a certified tank inspection is mandatory every 5 to 10 years depending on the type and local regulations.
Electronics and Sensors
The LPG ECU manages the entire system using data from several sensors, the reducer temperature sensor, the gas pressure sensor (MAP sensor), the gas temperature sensor and signals from the original petrol ECU. A problem on any one of these sensors affects the whole system.
Oxidised connectors on the injectors are a classic issue, especially on installations older than five to seven years. Engine vibration and heat crack wire insulation, and moisture finishes the job. A reducer temperature sensor giving wrong readings can make the system think the reducer is cold and refuse to switch to gas, or the opposite, switch too early while the reducer is still cold.
On older installations with emulators (devices that simulate the petrol injectors so the petrol ECU does not flag an error), a common problem is emulator calibration drifting after a spark plug or coil change. The emulator was set for certain parameters, and when those change the emulator can start generating false faults.
Switchover Problems
Switching from petrol to gas is not instant. It is a process that depends on several conditions being met: coolant temperature must reach a threshold (typically 35 to 45 degrees Celsius), reducer pressure must be stable, and the LPG ECU must receive clean signals from all sensors. When any of these conditions is not met the system stays on petrol or switches back.
"The car won't switch to gas" is one of the most common reasons for a workshop visit. The leading cause is a reducer temperature sensor that does not send an accurate value, so the ECU never gets the signal that the reducer is ready. The second common cause is low gas pressure from a clogged filter or a worn reducer membrane; the ECU registers unstable pressure and refuses the switch. A third situation is when the car does switch to gas but reverts to petrol after a minute or two. That usually means the LPG ECU detects excessive correction relative to the expected maps and returns to petrol as a protective measure.
If you are wondering whether the problem is fuel quality rather than the system itself, read our dedicated guide on that topic. And for a detailed look at what exactly gets checked during an LPG service, see our LPG inspection walkthrough.
Every one of these problems is solvable once properly diagnosed. If you notice any of the symptoms described above, or the gas simply does not perform the way it used to, come in for a check. We run full diagnostics and LPG system servicing for all installation types. Book a slot through our contact page.