Fuel Pump Secondary Circuit Low

What P0231 means

The fuel pump has two electrical sides. The primary (control) circuit is the low-current line the PCM uses to switch the relay or command a fuel pump control/driver module (that side is what P0230 covers). The secondary circuit is the switched battery-voltage feed that carries the working current from the relay or driver module to the pump motor. The PCM watches the voltage on that secondary feed — directly, or through a monitor/feedback line from the fuel pump control module — and when the pump is commanded to run it expects to see close to battery voltage there. P0231 sets when that secondary feed reads too LOW, which typically means an open, a high-resistance connection, or a short to ground somewhere in the pump power wiring, the relay or driver output stage, or the pump ground. On Chrysler and Jeep the relay lives inside the TIPM and GM systems use a fuel pump relay, while Ford's electronic returnless system reports secondary-side trouble back through the Fuel Pump Monitor (FPM) feedback (its own faults show as P025x), but the underlying idea is identical: the power path to the pump is not delivering the voltage the PCM expects.

Symptoms

• Engine cranks but will not start because the pump gets little or no power
• Sudden stalling or the engine dying while driving as the secondary feed drops out
• Hard starting or long cranking, often worse once the engine is hot
• Low or falling fuel pressure under load, causing hesitation, surging, or misfire-like stumble
• Check Engine light on, often with companion fuel-pump codes (P0230 control-side, P0232 secondary-high, or Ford's P025x)
• Weak or intermittent priming buzz from the tank, or no pump hum at all when the key is turned to RUN

Common causes

• Corroded, loose, or high-resistance connector or splice in the fuel pump power (secondary) feed
• Open, chafed, or broken wire in the secondary circuit between the relay/driver module and the pump
• Worn relay output contacts, or a failed driver output stage inside the fuel pump control module, FPDM, or TIPM
• Short to ground on the secondary feed that pulls the monitored voltage low
• Poor or corroded ground at the fuel pump or at the pump driver module
• Blown fuse or corroded power distribution feeding the relay/driver output, often from water intrusion in the TIPM or fuse box
• Worn, high-resistance fuel pump motor brushes causing an abnormal voltage drop on the feed (pump nearing failure)
• Internal PCM or driver fault on the fuel pump output monitor (least common — confirm wiring and pump first)

Severity & driving advice

Severity: High — A low secondary feed can starve or stop the fuel pump, causing a no-start or a stall in traffic.

Can I drive? Treat this as urgent. If the secondary power feed drops out the pump loses drive voltage, which can leave you with a no-start or cause the engine to stall while moving. If it still runs, keep trips short and get it diagnosed before it strands you.

Diagnostic approach

1. Read codes and freeze-frame first — Pull all stored DTCs and note the companions. P0231 is the secondary (power) side, so seeing P0230 alongside it points at the control circuit too, while P0232 is the same secondary circuit reading high instead of low. On Ford electronic returnless systems the equivalent faults appear as P025x, so identify the platform before you probe.
2. Confirm whether the pump actually runs and builds pressure — With the key turned to RUN, listen for the brief priming buzz (roughly 1-2 seconds) from the tank, then measure fuel pressure with a gauge or a scan-tool pressure PID and compare against the vehicle's spec. No prime and low or zero pressure supports a secondary power-feed fault; normal pressure suggests the fault is intermittent.
3. Measure voltage on the secondary feed at the pump — Back-probe the pump power (secondary) terminal with the pump commanded ON and compare to battery voltage. You should see close to battery level (about 12 V, or whatever the driver is currently commanding on variable-speed systems); a reading well below battery with the pump commanded on confirms the low-voltage condition P0231 is reporting.
4. Voltage-drop test the power feed and ground — With the pump running, measure the voltage drop along the secondary feed wire and along the pump ground path. More than roughly 0.5 V of drop on either leg indicates corrosion or a high-resistance connection choking the current — the classic cause of a low secondary reading with an otherwise good pump.
5. Test the relay or driver output — Confirm the fuel pump relay, FPDM, or TIPM actually delivers battery voltage onto the secondary circuit when commanded. Swapping in a known-good relay is a quick check on relay systems. On modules that report back (Ford FPM feedback), the module flags a secondary-circuit concern via its monitor duty cycle, so use that feedback to separate a bad driver output from bad downstream wiring.
6. Inspect the secondary wiring, connectors, and grounds end to end — Follow the power feed from the relay/driver to the tank connector and the pump ground. Look for backed-out or green-corroded terminals, chafed insulation, and water intrusion in the TIPM or underhood fuse box, which is a well-known trigger on Chrysler and Jeep vehicles.
7. Evaluate the pump and driver module last — If the secondary feed voltage and ground are good but pressure is still low, suspect a worn pump motor (high-resistance brushes drag the feed voltage down). Only after the wiring, feeds, grounds, relay, and pump all check out should you condemn the fuel pump control module or the PCM's internal driver, since those are the least common and most expensive causes.

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