This is a classic case of a shared electrical issue. When multiple sensors fail simultaneously, especially with "circuit malfunction high" or erratic readings (like flickering to 1000 degrees), it strongly points to a common power, ground, or reference voltage problem, rather than individual sensor failures. The fact that this occurred immediately after an auxiliary coolant pump replacement is also highly suspicious.
Let's break down the codes and what they indicate, then discuss common shared circuits and troubleshooting steps:
Understanding the Fault Codes:
- P218500: Engine Coolant Temperature Sensor 2 Circuit High - This means the ECU is seeing a voltage on the signal wire of ECT sensor 2 that is too high, often indicating an open circuit, a short to a positive voltage, or a missing ground.
- P203100: Exhaust Gas Temperature Sensor 2 Circuit (generic code, likely "circuit malfunction") - Similar to P218500, indicating an electrical issue with EGT sensor 2.
- P242A00: Exhaust Gas Temperature Sensor 3 Circuit (generic code, likely "circuit malfunction") - Same as above, for EGT sensor 3.
- P246E00: Exhaust Gas Temperature Sensor 4 Circuit (generic code, likely "circuit malfunction") - Same as above, for EGT sensor 4.
The flickering readings to 1000 degrees are a tell-tale sign of an open circuit or intermittent connection. When the ECU loses the signal from a thermistor-type sensor (which most of these are), it often defaults to a high reading to indicate a fault or prevent damage.
Common Shared Circuits and Where to Look:
Many sensors on modern vehicles, particularly temperature sensors, share common
5V reference voltage lines and
ground connections from the Engine Control Unit (ECU).
Given the symptoms, here's what your friend should focus on:
- Damaged Wiring / Connector at the Auxiliary Coolant Pump:
- This is the prime suspect. When replacing the pump, it's very easy to inadvertently pull, pinch, or damage wiring nearby.
- Check the wiring harness leading to and from the auxiliary coolant pumpvery carefully. Look for:
- Pinched or cut wires: Especially where the harness passes through grommets or near sharp edges.
- Loose or corroded connections: At the pump connector itself, and any adjacent connectors.
- Damaged insulation: Exposing bare wires that could be shorting.
- Check for damage to the main engine harness in that area. The auxiliary pump wiring often runs alongside other critical sensor wiring.
- Shared Ground Point:
- Multiple sensors often share a common ground point on the engine block or chassis. If this ground point is loose, corroded, or damaged, it can affect all sensors connected to it.
- Trace the ground wires from the affected sensors. This can be tricky without a wiring diagram, but look for brown wires (common ground color in VW) that lead to a bolted connection on the engine or chassis.
- A good wiring diagram for the CAAB engine would be invaluable here. Unfortunately, specific diagrams are hard to find publicly. VW workshop manuals (ElsaWin) would have this.
- Shared 5V Reference Voltage:
- The ECU supplies a 5V reference voltage to many sensors. If this reference voltage is disrupted, all sensors relying on it will fail.
- This is harder to check without knowing which pins carry the 5V reference. However, if the ground is good and the sensors still read high, a lost 5V reference is a strong possibility.
- ECU Connector/Pins:
- While less likely after a pump replacement, a severely disturbed wiring harness could strain the ECU connector.
- Check that the large multi-pin connectors on the ECU are fully seated and that no pins are bent or pushed out.
Troubleshooting Steps for Your Friend:
- Visual Inspection (Thoroughly!):
- Start at the auxiliary coolant pump area and systematically follow the wiring harness away from it.
- Look for any signs of damage: abrasions, cuts, pinches, burn marks, or loose connectors.
- Pay close attention to any areas where the harness might rub against engine components or the body.
- Continuity and Voltage Checks (with a Multimeter):
- Disconnect the battery before performing any checks that involve disconnecting connectors on active circuits.
- Identify the sensor wires: Each sensor typically has two wires for a thermistor (signal and ground) or three wires if it's a hall-effect or active sensor (power, signal, ground). EGT sensors are usually thermistors.
- Check for continuity to ground: With the sensor disconnected, check for continuity between the ground wire of each affected sensor and a known good chassis ground. If any show open, trace that ground wire.
- Check for 5V reference (with ignition on, engine off): With the sensor disconnected, identify the 5V reference wire (if applicable) and check for 5V with the ignition on. Be extremely careful not to short circuits when doing this. If there's no 5V where there should be, the problem is upstream.
- Check signal wire continuity to ECU: This is more advanced and requires a wiring diagram to know which pin on the ECU corresponds to which sensor signal wire. But if all else fails, this can help identify an open circuit in the signal wire.
- Wiggle Test:
- With the engine running and observing the Carscanner Pro readings, gently wiggle sections of the wiring harness, particularly around the auxiliary pump and where it leads towards the ECU. If the readings change or flicker more dramatically, you've found a likely spot of intermittent connection.
- Forced Regeneration & Fan Running:
- The DPF light and forced regen, followed by constant fan running, are all consequences of the EGT sensor faults. The ECU is likely seeing incorrect exhaust temperatures, preventing normal DPF operation, and then running the fan continuously as a safety measure due to perceived high temperatures. Fixing the sensor circuit issues should resolve these symptoms.
Why the auxiliary coolant pump replacement is key:
It's highly probable that during the replacement, a wire was pulled, caught, or damaged that shares the circuit with these sensors. The auxiliary coolant pump and its wiring are often in proximity to other engine components and harnesses, making accidental damage a real possibility.
If your friend can get access to an official VW workshop manual or wiring diagrams for the CAAB engine, it would greatly assist in identifying the specific wire colors and pinouts for the sensors and their shared power/ground. Lacking that, a methodical visual inspection and continuity/voltage checks will be the most effective approach.
