What causes a MAP sensor high input code?

On engines where the sensor is connected to the manifold by a hose, a split or disconnected hose is the most common cause: the sensor reads atmospheric pressure permanently, which at idle is impossibly high. Electrically, a signal wire shorted to a supply voltage or an open earth on the sensor circuit will both push the reading to the top of its range. A failed sensor and, rarely, a badly blocked exhaust account for most of the rest.

Can a blocked catalytic converter cause P0108?

Yes, though it is uncommon. A severely restricted exhaust stops the engine expelling gas properly, which raises the pressure left in the inlet manifold at idle. The sensor then reports a genuinely high figure and the control unit rejects it as implausible. It is worth remembering because it is the one cause where replacing the sensor changes nothing, and a back-pressure test settles it quickly.

Will P0108 fail an MOT?

Yes, and potentially twice. The illuminated engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures against it. Separately, the rich mixture that P0108 causes raises the carbon monoxide and hydrocarbon readings measured during the emissions check, so the exhaust gas result can fail on its own account.

Is P0108 dangerous to the engine?

It is not dangerous to drive, but it is damaging to leave. A permanently rich mixture overheats the catalytic converter, fouls the spark plugs, and washes lubricating oil off the cylinder bores while diluting the oil in the sump with fuel. None of that is immediate, but weeks of it turns a sensor job into a much larger one.

Powertrain code · standard

P0108Manifold pressure sensor reading too high

Standard definition: Manifold Absolute Pressure Sensor High Input

Last updated 27 July 2026

Quick answer

The manifold pressure sensor is reporting a pressure higher than the engine could be producing, so the control unit has discarded it. On a naturally aspirated engine that means the sensor thinks the manifold is at atmospheric pressure or above while the engine is idling.

How the car works this out

Manifold pressure is at its lowest with the engine idling and the throttle closed. The control unit sets P0108 when the signal sits above the ceiling it allows — typically above about 4.5 volts, or a pressure reading that could not occur at the throttle position and engine speed it is commanding. On a naturally aspirated engine, anything at or above barometric pressure while idling is impossible. On a turbocharged engine the limit is higher but the principle is identical.

What you might notice

Very rich running and a strong fuel smell
Black smoke from the exhaust
Rough idle, misfire or stalling
Poor fuel economy and sooty plugs
Reduced power or limp mode

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1The vacuum hose to a remotely mounted sensor split, perished or knocked off, so the sensor reads atmospheric pressure all the timeCommon
How to check: With the engine idling, the live reading should be well below atmospheric. If it is sitting at around 100 kPa at idle, the sensor is not seeing manifold vacuum at all — check the hose and its port.
2The signal wire shorted to the five-volt reference or to battery voltageCommon
3An open or high-resistance earth on the sensor circuitPossible
How to check: Measure the voltage drop between the sensor's earth pin and the battery negative terminal. More than a couple of tenths of a volt with the engine running is a poor earth.
4A failed MAP sensorPossible
5A severely restricted exhaust, which genuinely raises manifold pressure at idleLess common
How to check: A back-pressure test at the oxygen sensor port. This is the rare cause but the important one, because replacing sensors will never fix it.

What to do about it

1Find the sensor and check whether it is mounted directly on the manifold or connected by a hose. If there is a hose, that hose and its port are the first thing to examine.
2Watch the live reading at idle. A value near atmospheric pressure with a running engine tells you the sensor is not seeing vacuum, whether because of a hose or because it has failed.
3Check the sensor's earth before condemning it. A lost earth pushes the signal high with no fault in the sensor at all.
4Deal with it rather than tolerating it. The control unit is being told the engine is under load when it is not, so it is pouring in fuel, which washes the bores, dilutes the oil and cooks the catalytic converter.

Can you keep driving?

Briefly, gently

It will drive, but it is running rich the entire time and that is doing damage on a schedule. Unburnt fuel reaching a hot catalytic converter is the expensive failure mode here, and fuel washing past the rings into the sump is the slower one. Keep it short.

Will it fail an MOT?

Beyond the lamp itself, which is a major defect with 371,859 recorded failures, P0108 is one of the more reliable ways to fail the emissions test. The over-rich mixture it produces drives carbon monoxide and hydrocarbons straight up on a petrol car, and if the catalytic converter has been damaged by weeks of rich running, the emissions failure will remain after the sensor is replaced.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

What causes a MAP sensor high input code?: On engines where the sensor is connected to the manifold by a hose, a split or disconnected hose is the most common cause: the sensor reads atmospheric pressure permanently, which at idle is impossibly high. Electrically, a signal wire shorted to a supply voltage or an open earth on the sensor circuit will both push the reading to the top of its range. A failed sensor and, rarely, a badly blocked exhaust account for most of the rest.
Can a blocked catalytic converter cause P0108?: Yes, though it is uncommon. A severely restricted exhaust stops the engine expelling gas properly, which raises the pressure left in the inlet manifold at idle. The sensor then reports a genuinely high figure and the control unit rejects it as implausible. It is worth remembering because it is the one cause where replacing the sensor changes nothing, and a back-pressure test settles it quickly.
Will P0108 fail an MOT?: Yes, and potentially twice. The illuminated engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures against it. Separately, the rich mixture that P0108 causes raises the carbon monoxide and hydrocarbon readings measured during the emissions check, so the exhaust gas result can fail on its own account.
Is P0108 dangerous to the engine?: It is not dangerous to drive, but it is damaging to leave. A permanently rich mixture overheats the catalytic converter, fouls the spark plugs, and washes lubricating oil off the cylinder bores while diluting the oil in the sump with fuel. None of that is immediate, but weeks of it turns a sensor job into a much larger one.

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