The engine ECU has failed its own internal memory check. Genuine ECU failure is rarer than people think — check power and earths first.
P0605 — also written PO6O5, P 0605, 0605 — is the OBD-II code for internal control module rom error. Same code, same fix: this page covers every spelling of it.
P0605 is a electrical fault code, typically costing £150–£1600 to put right in a UK garage.
What causes P0605?
- Corrupted ECU software after a flat battery or bad jump start
- Water ingress into the ECU housing
- Poor earth causing voltage spikes
- Genuine internal ECU failure
How P0605 is fixed
- Check ECU supply and earth integrity
- Try a software re-flash with the latest calibration
- Use a specialist ECU repair service before buying new — usually far cheaper
- Replace and code the ECU as a last resort
P0605 repair cost in the UK
Most P0605 repairs land between £150–£1600 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Can you drive with P0605?
May not start or may cut out. Recover rather than risk it.
Urgency: high.
What P0605 is telling you
This is a wiring-first code. The ECU sees an impossible voltage on the circuit, and until the loom, connector and earth path have been checked, replacing the component is a gamble.
On P0605 specifically — internal control module rom error — the stored code records the moment that check failed, not the moment the part gave up. That gap is why two cars with the same code can need completely different repairs.
How P0605 is diagnosed properly
Each of the known causes is ruled in or out in cost order, cheapest and most likely first. On P0605 that means working through them like this:
- Corrupted ECU software after a flat battery or bad jump start — checked for outstanding manufacturer updates before any hardware is touched.
- Water ingress into the ECU housing — opened, inspected for green corrosion and spread pins, and back-probed rather than unplugged, so the reading is taken with the circuit live.
- Poor earth causing voltage spikes — confirmed with live data before any part is bought.
- Genuine internal ECU failure — checked for outstanding manufacturer updates before any hardware is touched.
An hour of diagnostic time on P0605 routinely saves more than it costs, because the alternative is buying the most expensive part on that list and hoping.
The repairs that actually clear P0605
- Check ECU supply and earth integrity
- Try a software re-flash with the latest calibration
- Use a specialist ECU repair service before buying new — usually far cheaper
- Replace and code the ECU as a last resort
After the repair the code is cleared and the car is driven until the relevant readiness monitor completes, rather than handed back as soon as the light goes out. A code that has only been erased will come straight back on the next drive cycle.
What happens if P0605 is ignored
This is a stop-and-sort fault rather than a wait-for-the-service fault. Ignoring it risks turning a sensor-money repair into a component-money one.
For reference, P0605 typically lands at £150–£1600 all-in at an independent garage in the UK — main-dealer quotes for the same job usually sit above that.
Getting P0605 looked at
Bob's Mechanical Repairs is an independent garage in Birnam, Dunkeld, Perthshire, with 32 years of Bob's experience on the tools. If P0605 is showing on your car, bring the scan results with you — we will read the live data ourselves, tell you which of the causes above it actually is, and price the repair before any work starts.