Volvo B12B DH12E - multiple sensor DTCs, rough running and engine light

AidenWilson11 wrote:

Faults on b12b dh12e engine

Chassis ID: B12B 146686 VIN: YV3R8R420BA146686
DTC
PID 100: Engine oil pressure, FMI Voltage above normal or shorted high
PID 107: Air filter pressure drop, FMI 5: Cu...

Initial assessment of available information

The fault list on your Volvo B12B / DH12E does not look like nine unrelated component failures. The pattern is strongly electrical and points to a common supply, common sensor ground, ECU connector, or ECU internal fault.

You have already found several important clues:

- Oil contamination in the common 5 V sensor feed/harness.
- Spread ECU connector pins.
- Previous poor mechanical repair: a bolt wedged into/around the ECU plug locking mechanism.
- Harness removed and repaired.
- After clearing, most faults return active.
- Engine runs rough and engine light is on.

That combination makes me suspicious of:

1. Poor ECU connector engagement/contact pressure.
2. Open or high-resistance analogue sensor ground.
3. Damaged common 5 V / sensor ground splice inside the engine harness.
4. Internal ECU damage, especially if 24 V or battery voltage has previously entered the 5 V sensor circuit.
5. Less likely, multiple failed sensors, but that should not be assumed first.

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Direct answer to the question

No, I would not start by replacing all listed sensors. The DTC pattern suggests a shared electrical fault.

The most likely area is the ECU connector and the common sensor supply/ground network. The bolt in the ECU plug locking mechanism is a major warning sign. If the ECU plugs are not being pulled squarely and fully into the ECU, you can get exactly this type of mixed FMI 3 and FMI 5 fault list.

Before going further, repair/replace the ECU connector locking housing so that the plug locks correctly. Then measure the circuits with a multimeter under load, not only with a diagnostic tool.

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Detailed problem analysis

1. Meaning of the FMI pattern

Your codes can be divided into two main groups.

Code	Circuit	FMI	Meaning	Likely electrical interpretation
PID 100	Engine oil pressure	FMI 3	Voltage above normal / shorted high	Signal near 5 V or supply voltage
PID 94	Fuel delivery pressure	FMI 3	Voltage above normal / shorted high	Signal too high
PID 153	Crankcase pressure	FMI 3	Voltage above normal / shorted high	Signal too high
PID 111	Coolant level	FMI 3	Voltage above normal / shorted high	Signal too high/open depending sensor type
PID 107	Air filter pressure drop	FMI 5	Current below normal/open circuit	Open circuit or missing supply/return
PID 110	Coolant temperature	FMI 5	Current below normal/open circuit	Open sensor, open signal, open return
PID 172	Air inlet temperature	FMI 5	Current below normal/open circuit	Open sensor, open signal, open return
PPID 124	EPG 1	FMI 5	Current below normal/open circuit	Open solenoid circuit or missing 24 V feed
PPID 6	Key-off input sensor	FMI 5	Current below normal/open circuit	Open input circuit/feed/return

This mix is important.

FMI 3 on pressure sensors

Oil pressure, fuel pressure and crankcase pressure sensors are normally 3-wire analogue sensors:

- 5 V reference feed,
- sensor ground/analogue ground,
- signal wire back to ECU.

If the sensor ground is missing, the signal can float high. The ECU then sees a high signal voltage and logs FMI 3.

If the 5 V feed is shorted to the signal wire, the ECU also sees FMI 3.

If the ECU connector pin for sensor ground is not contacting properly, several sensors can fail together.

FMI 5 on temperature and solenoid circuits

Coolant temperature and air inlet temperature sensors are usually 2-wire thermistors. FMI 5 usually means the ECU sees an open circuit or current too low.

Possible causes:

- broken wire,
- poor ECU pin contact,
- open common sensor return,
- failed sensor,
- internal ECU pull-up/pull-down circuit damage.

For the EPG solenoid, FMI 5 normally means open circuit or no current through the coil. That may be:

- open solenoid winding,
- no 24 V supply to the solenoid,
- broken wire,
- poor ECU driver connection,
- failed ECU output driver.

But because you also have many sensor faults, the EPG should be treated as part of a broader harness/ECU connector problem until proven otherwise.

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The bolt in the ECU connector is not acceptable

The bolt placed in the ECU plug/locking mechanism is a critical finding.

The ECU connector locking mechanism is designed to pull the connector into the ECU evenly and with correct pin pressure. If someone has used a bolt to “add tension” or jam the plug:

- the plug may sit crooked,
- some pins may touch intermittently,
- female terminals may become spread,
- ECU male pins may be bent,
- vibration may cause changing faults,
- outer-row pins may lose contact first,
- water/oil sealing may be compromised.

With multiple active faults returning immediately, you need to restore the ECU connector mechanically before deeper diagnostics. Otherwise, your voltage readings may be misleading.

Recommendation: replace the damaged connector body/locking mechanism and any suspect female terminals. Do not rely on wedging, cable ties, extra bolts or pressure tricks on an engine ECU connector.

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Practical diagnostic procedure

Do this systematically. “Most voltages are ok” is not enough here. You need actual numbers at the ECU and at the sensors.

Step 1: Confirm which codes are active after clearing

1. Key off.
2. Disconnect batteries for a short period if appropriate for your workshop procedure.
3. Reconnect.
4. Clear all DTCs.
5. Key on, engine off.
6. Read active faults.
7. Start engine if possible.
8. Read active faults again.

Write them down as:

DTC

KOEO active?

Engine running active?

Comes back immediately?

This matters because some codes may be old stored faults from the harness repair.

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Step 2: Check 5 V reference properly

At one of the affected 3-wire sensors, for example engine oil pressure:

With ignition ON, sensor unplugged:

Measurement	Expected result
5 V reference to sensor ground	About 4.8–5.1 V
5 V reference to battery negative	About 4.8–5.1 V
Sensor ground to battery negative	Ideally less than 0.1–0.3 V voltage drop under load
Signal wire to sensor ground	Depends on ECU design, often a bias voltage, not battery voltage

Important interpretation:

- If you get 5 V from 5 V pin to battery negative, but 0 V or unstable voltage from 5 V pin to sensor ground, then the sensor ground is open.
- If 5 V is much higher than 5.2 V, suspect ECU regulator fault or wrong voltage entering the circuit.
- If 5 V disappears when sensors are plugged in, one sensor or branch may be shorting the reference.

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Step 3: Load-test the sensor ground

A continuity test alone is not enough. A damaged wire or bad terminal can show continuity with no load but fail under current.

Use a small test load, for example a 12 V/24 V suitable test bulb is too heavy for ECU sensor ground, so use a controlled low current load such as:

- 1 kΩ resistor from 5 V reference to sensor ground.

This draws about:

$$ I = \frac{5V}{1000\Omega} = 5mA $$

Then measure voltage drop on the sensor ground back to battery negative.

Expected:

- Sensor ground should remain near 0 V.
- Voltage drop should be very low, preferably below 0.05–0.1 V for such a small load.

If sensor ground rises significantly, you have a high-resistance return, bad splice, bad ECU connector terminal, or ECU internal ground fault.

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Step 4: Compare at sensor and ECU pins

You need to know whether the fault is:

- in the sensor,
- between sensor and ECU,
- at the ECU connector,
- inside the ECU.

For each affected sensor, measure at:

1. Sensor plug.
2. Back-probed ECU connector, if possible.
3. ECU harness connector unplugged, using continuity tests.

Tests to perform with ECU disconnected and batteries isolated:

Test	Expected
Sensor 5 V wire to corresponding ECU pin	Near 0 Ω
Sensor ground wire to ECU sensor ground pin	Near 0 Ω
Signal wire to ECU signal pin	Near 0 Ω
Each wire to chassis/battery positive	Open circuit
Each wire to neighbouring wire	Open circuit unless common splice intended
Wiggle test while measuring	No dropouts

If the harness tests good to the ECU plug, but faults remain and measured ECU output/input behaviour is wrong, then suspect:

- poor ECU terminal fit,
- damaged ECU pins,
- internal ECU damage.

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Step 5: Terminal tension test

Because you found spread ECU pins/terminals, you must verify terminal grip.

Use the correct terminal test pin or a known-good male terminal of the same size. Do not use oversized probes because they will damage the terminal again.

Check:

- female terminal grip,
- terminal locked into housing,
- no pushed-back terminals,
- no oil inside connector,
- no green corrosion,
- no fretting marks,
- no bent ECU pins,
- no cracked connector housing,
- locking slide pulls plug fully home.

A terminal can look acceptable but fail under vibration.

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Likely common-cause scenarios

Scenario A: Open analogue sensor ground

This is my first suspect because it explains the pressure sensor FMI 3 faults well.

Symptoms:

- Multiple 3-wire sensor signals read high.
- Diagnostic live values may show unrealistic maximum pressure.
- Faults return immediately.
- Engine may run rough due to incorrect fuel pressure, oil pressure, temperature or air data.

Confirm by measuring:

- 5 V reference to battery negative = OK,
- 5 V reference to sensor ground = not OK,
- sensor ground to battery negative = high resistance/open.

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Scenario B: 5 V reference contaminated or shorted

Oil in the harness can cause leakage between conductors. It can also wick into ECU connectors. Even after repair, oil residue can remain inside connector housings.

Symptoms:

- Multiple 5 V sensors faulting.
- 5 V reference unstable or pulled up/down.
- Faults change when harness is moved.
- Some sensors read implausible values.

Confirm by:

- unplugging all 5 V sensors,
- checking 5 V stability,
- plugging sensors in one at a time,
- insulation testing the harness with ECU and sensors disconnected.

Do not megger the ECU or sensors. If using insulation testing, disconnect all electronics and use an appropriate voltage for the harness only.

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Scenario C: ECU plug not seated squarely

This is highly plausible because of the bolt.

Symptoms:

- Many unrelated DTCs.
- Codes return after movement/vibration.
- Spread pins or pushed-back terminals.
- Engine may run rough.
- Some measurements “seem ok” when stationary but faults occur running.

Confirm by:

- visual inspection,
- connector replacement,
- wiggle test,
- back-probing while moving the plug gently,
- checking terminal tension.

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Scenario D: ECU internal damage

If battery voltage has entered the 5 V reference or sensor ground circuit, the ECU may have damaged:

- 5 V regulator,
- analogue ground track,
- input protection components,
- pull-up resistors,
- output drivers.

Symptoms:

- Harness tests good.
- Sensor supply/ground incorrect directly at ECU.
- Multiple unrelated inputs fail.
- Same codes remain with sensors disconnected or with substitute resistors.
- ECU may have oil inside or previous overvoltage damage.

Confirm by:

- testing harness disconnected,
- checking ECU pin outputs directly with correct loads,
- substituting a known-good ECU if immobiliser/programming situation allows,
- sending ECU for professional bench test.

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Important live-data checks

Do not only read DTCs. Look at live values.

With key on/engine off, compare live readings:

Parameter	Normal expectation KOEO
Engine coolant temp	Near ambient temperature
Air inlet temp	Near ambient temperature
Oil pressure	Zero or near zero, depending system
Fuel delivery pressure	Plausible low/prime pressure
Crankcase pressure	Near atmospheric/zero differential
Coolant level	Correct state, not flickering
Air filter restriction	Low/normal with engine stopped

If coolant temperature shows -40 °C or extremely low, that usually indicates open circuit on an NTC sensor. If it shows very high temperature, that may indicate short circuit, depending system logic.

If pressure values show maximum, that supports a high signal/open ground/short to 5 V condition.

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Testing temperature sensors with resistors

For 2-wire NTC sensors such as coolant temperature and air inlet temperature, you can use a resistor substitution test.

With the sensor unplugged, insert a resistor across the harness connector pins:

Test resistor	Expected behaviour
---:	---
Around 2.2 kΩ to 3.3 kΩ	ECU should display a moderate temperature, often near room-temperature range depending calibration
Open circuit	Very cold/implausible and FMI 5 likely
Short circuit	Very hot/implausible and different FMI likely

The exact temperature depends on Volvo’s calibration, but the key point is: if a reasonable resistor makes the live data become plausible and the fault inactive, the ECU and wiring are probably able to read that circuit. If nothing changes, suspect wiring/ECU connector/ECU.

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EPG PPID 124 FMI 5 check

The EPG fault may be separate, but check it after the ECU connector and sensor grounds.

At the EPG solenoid:

1. Key on.
2. Check for 24 V supply on one side of the solenoid, if this system uses a switched positive feed.
3. Check coil resistance.
4. Check wiring from solenoid to ECU driver.
5. Check for open circuit, corrosion or broken wire near the exhaust/heat area.

If the coil is open, replace the EPG solenoid. If the coil and feed are good but the ECU still sees open circuit, suspect ECU driver wiring or ECU pin contact.

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What I would do next, in order

1. Remove the bolt workaround permanently.
2. Repair/replace the ECU connector housing and locking mechanism.
3. Replace any questionable female terminals again using correct terminals and crimp tooling.
4. Clean oil from ECU plugs and harness plugs using suitable electrical cleaner; inspect sealing.
5. Clear DTCs and see what returns.
6. Measure and record:
- 5 V reference to sensor ground,
- 5 V reference to battery negative,
- sensor ground to battery negative,
- signal voltage on oil pressure, fuel pressure and crankcase pressure.
7. Unplug all sensors on the common 5 V line; reconnect one at a time.
8. Load-test the 5 V and sensor ground circuits.
9. Check continuity from affected sensors to ECU pins using wiring diagram.
10. If harness and connector prove good, have the ECU bench-tested or substitute a known-good programmed ECU.

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Do not assume “most of them are ok”

For this type of fault, exact values are essential. Please post readings like this:

Circuit	At sensor	At ECU	KOEO	Running
Oil pressure 5 V to sensor ground	V	V
Oil pressure 5 V to battery negative	V	V
Oil pressure sensor ground to battery negative	V drop / Ω	V drop / Ω
Oil pressure signal	V	V
Fuel pressure signal	V	V
Crankcase pressure signal	V	V
Coolant temp voltage across pins	V	V
Inlet temp voltage across pins	V	V

That will quickly show whether this is a shared ground/reference fault or ECU damage.

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Brief summary

Your fault list points to a common electrical issue, not multiple bad sensors. Given the history of oil in the 5 V harness, spread ECU pins and a bolt wedged in the ECU connector, the highest-probability faults are:

- bad ECU connector locking/contact,
- open/high-resistance analogue sensor ground,
- damaged common 5 V/sensor ground splice,
- or internal ECU damage.

Repair the ECU connector properly first, then perform exact voltage and load tests on the 5 V reference, sensor ground and signal circuits. If the harness and connector test correctly but the codes still return active, the ECU likely needs professional bench testing or replacement/programming.