Symptom
Vehicle will not switch over to gas
A converted or factory bi-fuel car stays on petrol and never makes the changeover, or drops back to petrol shortly after making it. Every one of these systems starts on petrol deliberately and switches only once conditions are met — usually a coolant temperature, because the vaporiser needs engine heat to turn liquid gas into vapour, and sometimes a minimum engine speed. So a car that will not switch on a cold morning may be working perfectly, and one that will not switch when hot is not. The changeover conditions are the first thing to establish, because half of these complaints turn out to be the system refusing for a reason.
Also described as: wont switch to gas, stays on petrol, lpg not switching, wont run on lpg, cng not working, gas system not switching over, autogas wont engage, روی گاز نمیرود, دوگانه سوز روی گاز نمیرود, گاز عمل نمیکند, no cambia a gas, no pasa a gas, no entra gas, sigue en gasolina, no cambia a glp, no cambia a gnc, لا تتحول للغاز, ما تشتغل على الغاز, تبقى على البنزين
What this usually means
Establish the changeover conditions before diagnosing a failure, because a bi-fuel system that refuses is very often working correctly. Every one of these installations starts on petrol deliberately and switches only once its conditions are met — the important one being a vaporiser temperature, because the vaporiser uses engine coolant heat to turn liquid gas into vapour and cannot do its job cold. Some systems also require a minimum engine speed, a minimum tank pressure, or a settled period after start.
So a car that will not switch on a cold morning may be entirely healthy, and one that will not switch when thoroughly warm is not. That is the first fork and it costs nothing.
When it genuinely will not switch, German LPG references give a diagnostic order worth following. The solenoid valves are the first thing: there is one at the tank and one before the vaporiser, and if they do not open the system cannot run on gas whatever else is correct. They are testable by ear — force a changeover with the ignition on and listen for the valves clicking. A system that will not switch and whose valves are silent is an electrical fault; one whose valves click and still will not switch is a supply or pressure fault, and that single observation splits the work.
Then pressure and filtration. A clogged gas filter, a vaporiser membrane that has aged and can no longer hold pressure, or a low tank level all starve the supply. And the temperature sensor reporting vaporiser temperature deserves its own suspicion: if it reads colder than reality, the system waits forever for a condition that has already been met — which presents exactly as a refusal to switch and is a sensor rather than a gas fault.
Read the gas system's own fault memory. It is separate from the engine management's, and a generic scan tool will not see it.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Will not switch on a cold engine, switches normally once warm | Nothing — the vaporiser needs coolant heat, and the system is waiting for its temperature condition | Note the coolant temperature at which it does switch and compare against the installation's threshold. Consistency is the evidence. | No repair. Explain the condition rather than diagnosing it. |
| Never switches, and the solenoid valves are silent when forced | Electrical — the valves are not being commanded, or their supply is absent | Force a changeover with the ignition on and listen at the tank valve and the one before the vaporiser: a healthy pair click audibly after a few seconds. Silence localises it to the electrical side. | Repair the supply, the fuse or the control output; renew the coil where it has failed open. |
| Valves click but the engine will not run on gas | Supply or pressure — a clogged filter, a low tank level, or a vaporiser that cannot hold pressure | Check pressure in the line between vaporiser and filter. Inspect the gas filter, which is a service item routinely forgotten. | Renew the filter, or the vaporiser membrane where pressure will not hold. Check the tank level and multivalve first. |
| Switches over then drops back to petrol shortly afterwards | Pressure falling under demand, or the gas management detecting a fault and reverting deliberately | Read the gas system's own fault memory — separate from the engine management's and invisible to a generic tool. Watch gas pressure live under load. | Repair what the gas system's codes name. A deliberate reversion is a protection, not a glitch. |
| Never switches, thoroughly warm, valves and pressure both fine | The vaporiser temperature sensor reading colder than reality, so a condition already met is never recognised | Compare the reported vaporiser temperature against the actual coolant temperature. A large disagreement on a warm engine is the fault. | Renew the sensor or repair its connector. This is a sensor fault presenting as a gas fault. |
| Stopped switching after other work on the car | A gas system connection disturbed, or coolant flow to the vaporiser interrupted after a cooling system repair | Retrace the work. Check the vaporiser's coolant hoses are connected and flowing, and that air was purged from the system. | Restore the connection and bleed the cooling system. An air-locked vaporiser never reaches temperature. |
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Will not switch on a cold engine, switches normally once warm
- Likely cause
- Nothing — the vaporiser needs coolant heat, and the system is waiting for its temperature condition
- How to confirm it
- Note the coolant temperature at which it does switch and compare against the installation's threshold. Consistency is the evidence.
- What fixes it
- No repair. Explain the condition rather than diagnosing it.
-
Never switches, and the solenoid valves are silent when forced
- Likely cause
- Electrical — the valves are not being commanded, or their supply is absent
- How to confirm it
- Force a changeover with the ignition on and listen at the tank valve and the one before the vaporiser: a healthy pair click audibly after a few seconds. Silence localises it to the electrical side.
- What fixes it
- Repair the supply, the fuse or the control output; renew the coil where it has failed open.
-
Valves click but the engine will not run on gas
- Likely cause
- Supply or pressure — a clogged filter, a low tank level, or a vaporiser that cannot hold pressure
- How to confirm it
- Check pressure in the line between vaporiser and filter. Inspect the gas filter, which is a service item routinely forgotten.
- What fixes it
- Renew the filter, or the vaporiser membrane where pressure will not hold. Check the tank level and multivalve first.
-
Switches over then drops back to petrol shortly afterwards
- Likely cause
- Pressure falling under demand, or the gas management detecting a fault and reverting deliberately
- How to confirm it
- Read the gas system's own fault memory — separate from the engine management's and invisible to a generic tool. Watch gas pressure live under load.
- What fixes it
- Repair what the gas system's codes name. A deliberate reversion is a protection, not a glitch.
-
Never switches, thoroughly warm, valves and pressure both fine
- Likely cause
- The vaporiser temperature sensor reading colder than reality, so a condition already met is never recognised
- How to confirm it
- Compare the reported vaporiser temperature against the actual coolant temperature. A large disagreement on a warm engine is the fault.
- What fixes it
- Renew the sensor or repair its connector. This is a sensor fault presenting as a gas fault.
-
Stopped switching after other work on the car
- Likely cause
- A gas system connection disturbed, or coolant flow to the vaporiser interrupted after a cooling system repair
- How to confirm it
- Retrace the work. Check the vaporiser's coolant hoses are connected and flowing, and that air was purged from the system.
- What fixes it
- Restore the connection and bleed the cooling system. An air-locked vaporiser never reaches temperature.
Codes that produce this symptom (90)
Listed with the codes that most typically present this way first — the likeliest place to start.
- P2A22 Alternative Fuel Tank Shutoff Valve B Control Circuit/Open Typical of this code High
- P2A23 Alternative Fuel Tank Shutoff Valve B Control Circuit Performance/Stuck Off Typical of this code High
- P2A24 Alternative Fuel Tank Shutoff Valve B Control Circuit Low Typical of this code High
- P2A25 Alternative Fuel Tank Shutoff Valve B Control Circuit High Typical of this code High
- P2A26 Alternative Fuel Tank Shutoff Valve C Control Circuit/Open Typical of this code High
- P2A27 Alternative Fuel Tank Shutoff Valve C Control Circuit Performance/Stuck Off Typical of this code High
- P2A28 Alternative Fuel Tank Shutoff Valve C Control Circuit Low Typical of this code High
- P2A29 Alternative Fuel Tank Shutoff Valve C Control Circuit High Typical of this code High
- P2A2A Alternative Fuel Tank Shutoff Valve D Control Circuit/Open Typical of this code High
- P2A2B Alternative Fuel Tank Shutoff Valve D Control Circuit Performance/Stuck Off Typical of this code High
- P2A2C Alternative Fuel Tank Shutoff Valve D Control Circuit Low Typical of this code High
- P2A2D Alternative Fuel Tank Shutoff Valve D Control Circuit High Typical of this code High
- P2A2E Alternative Fuel Tank Shutoff Valve E Control Circuit/Open Typical of this code High
- P2A2F Alternative Fuel Tank Shutoff Valve E Control Circuit Performance/Stuck Off Typical of this code High
- P2A30 Alternative Fuel Tank Shutoff Valve E Control Circuit Low Typical of this code High
- P2A31 Alternative Fuel Tank Shutoff Valve E Control Circuit High Typical of this code High
- P2A40 Alternative Fuel System - Injector Leakage to Rail Detected Typical of this code High
- P2A41 Alternative Fuel Tank B Pressure Sensor Circuit Typical of this code High
- P2A42 Alternative Fuel Tank B Pressure Sensor Range/Performance Typical of this code High
- P2A43 Alternative Fuel Tank B Pressure Sensor Circuit Low Typical of this code High
- P2A44 Alternative Fuel Tank B Pressure Sensor Circuit High Typical of this code High
- P2A45 Alternative Fuel Tank B Pressure Sensor Circuit Intermittent/Erratic Typical of this code High
- P2A46 Alternative Fuel Tank C Pressure Sensor Circuit Typical of this code High
- P2A47 Alternative Fuel Tank C Pressure Sensor Range/Performance Typical of this code High
- P2A48 Alternative Fuel Tank C Pressure Sensor Circuit Low Typical of this code High
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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